Car will "pulse/surge" when at cruising speed or

Engine check light on? Check all vacuum lines for leakage and check fuel to air sensors.

Is the car equipped with the Automatic Transmission?
If so, read on.
Otherwise skip to the end and answer some questions.

POSSIBLE SCENARIO:
I have observed a condition where my car surges slightly when the torque-converter clutch (TCC) cycles between lock and unlock when driving on an uphill grade.
First some basics and history that will explain why the TCC is used.

Engine, Torque Converter, TCC, and Transmission relationship--
The TCC allows for a solid connection between the engine and transmission which allows the input to the transmission to rotate at the same speed as the engine.
Without a TCC, there is slippage between the engine and automatic transmission. The slippage is greatest at low engine RPM. That is what allows the engine to run with the automatic transmission in gear, like when you first shift into gear or stop at a stop sign. When the throttle pedal is depressed, the engine RPM begins to increase and the torque converter begins to slip less and less the more the engine RPM increases. The car moves. But even at cruising speeds the torque converter slips slightly. Engine RPM is greater than transmission input RPM, which is realized as slight decrease in fuel efficiency.
When acceleration is complete and a constant speed is being maintained, the engine power output is reduced to the point where the TCC can engage and eliminate any slippage between the engine and transmission. If the car has a tachometer the engagement of the TCC can be verified when a slight reduction in engine RPM observed without a corresponding change in vehicle speed.
One method used to test the operation of the TCC is as follows:
Find a flat section of road where it is safe to perform the test.
Reach a steady speed and keep the gas pedal depressed with one foot. While observing the tachometer (or listening for an increase in engine RPM), with the other foot depress the brake pedal enough to activate the break light switch but not enough to engage the brakes. When the brake light switch activates, the TCC receives a signal to disengage. With the gas pedal being held steady, release the brake pedal and the engine RPM should decrease when the TCC engages.
Old cars with Automatic Transmissions did not use a TCC. I believe the TCC was put in use in an attempt to increase fuel economy.

MY EXPERIENCE WITH SIMILAR SYMPTOMS
The condition that causes that issue on my car is this:
- A slight uphill grade increases the load on the engine.
The car tends to gradually slow and it is necessary to depress the gas pedal to maintain speed.
- Depressing the throttle pedal (manually, or automatically with cruise control engaged) signals the torque converter clutch to unlock when the load increases slightly. (A more drastic load increase would signal the Transmission to downshift to a lower gear.) The corresponding increase in engine RPM and output is enough to compensate for the reduction in speed. When the vehicle speed, engine RPM, and throttle position stabilize to the point that the TCC will engage and the engine RPM will reduce in correspondence with TCC engagement. Now, if the road conditions have not changed, power output is not enough to maintain vehicle speed. With the increased load caused by full engagement between engine and transmission, and the cycle (surging) repeats itself until the road conditions change.

Does that help?
If not:

QUESTIONS
Please define the symptoms.
What are the road conditions when the surge occurs? (A slight uphill grade?)
What is the frequency of the surge?
Does the engine power output have a noticeable surge?
Is there a speed change related to the surge?
Does the tachometer move up and down with little or no change in vehicle speed?
Are all instrument indication in the normal range?
What else has changed?

Good luck!

check you MAP sensor.

First I'll assume you haven't read the owners Manuel, Cruise control is not perfect on any vehicle. if coming to a hill put foot on gas to help rpm, going down hill put foot on brake to keep same speed, cruise control operates off of engine vacuum. Speed sensors can be affected by putting on smaller or bigger tires than what is on drivers door.

What you are describing is a vacuum leak in one of the vacuum lines for the cruise control or a bad cruise control vacuum servo. I myself am a former owner of a 1979 Porsche 928 5 speed, a very beautiful and extremely powerful stock automobile. Good luck.

ENGINE SURGING COULD BE WEAK FUEL PUMP, STOPPED UP FUEL FILTER,BAD TORQUE CONVERTER SOLENOID OR TORQUE CONVERTER. OR FAULTY TPS.

possible mass air flow sensor fault, dissconnect it and see if it improves slightly to confirm or have scan for trouble codes

Is this occuring at highway speeds or normal cruising speeds??

Call the dealer who sold you this car while it is still under warranty and insist that they repair it. The cruise control is quite a complex system and a scan is required to isolate the problem.

If you care to learn a bit more about the system read on:
Cruise control systems are comprised of electronic and mechanical subsystems. This is how they work.
We all know that the things that control the speed of the car are the gas pedal and the brakes. And the brain that normally controls the speed of the car is the brain of the driver. The driver senses the speed by looking at the speedometer and then adjusting the pressure on the gas pedal or the brakes to compensate for variations in the desired speed. The cruise control system does the same thing with one exception. It only controls the gas pedal - it doesn't even know there are brakes in the car!!
The vehicle's speed sensor which is mounted on the output shaft of the transmission (the thing that drives the wheels) sends electrical pulses to the computer, pulses which are generated by a magnet spinning past a sensor coil. When the vehicle's speed increases the frequency of the pulses increases. For any given speed of the vehicle there is a corresponding pulse frequency. It is this pulse frequency which the cruise control tries to maintain as a constant. You think of it as the vehicle's speed.
The brains of the control box of the cruise control has three functions. First, it stores the speed of the vehicle when you press the "set" button whild travelling at the desired speed. It keeps this value in its memory until you turn the ignition off. Second, it receives the pulses from the transmission sensor and compares the frequency of those pulses to the frequency value stored in its memory - the set point. Third, it sends pulses to a vacuum controlled diaphragm connected to the accelerator linkage. The pulses it sends regulates the amount of vacuum the diaphragm receives. The more pulses, the more vacuum and the more vacuum the more force on the accelerator linkage. The system continues to add vacuum force until the set point speed is reached. At that point the system modulates the amount of vacuum the diaphragm receives in an effort to maintain the number of pulses coming from the speed sensor as close to the stored value as possible.
OK, so this "brain" works just fine in controlling the speed of the vehicle until something goes wrong. What can go wrong?
First, the VSS, the thing that sends pulses to the brain might fail. Normally the speedometer also fails so that's pretty easy to diagnose.
Next, the power to the brain can be interrupted. A blown fuse or a corroded connector can prevent the brain from working correctly or at all.
Next, the brains can lose its ability to function. A faulty component can prevent the brain from doing its thing. The brain is a pretty sophisticated box that contains a lot of electronic components including a microprocessor. NOrmally when the brains fail you need to replace the box..
The vacuum diaphragm can develop a leak. If that happens then the cruise control might set and hold the speed for some time however if the leak is larger than the supply line and modulator can add vacuum to the system the system will slowly lose control and the vehicle will slow down. This can also happen if the vacuum line to the diaphragm is cracked or loose.
Finally, the linkage that connects the diaphragm to the accelerator linkage can fail. Some aftermarket cruise control systems use a short length of what looks like fat key chain - bead chain. I have seen several units fail when the chain simply breaks.
Diagnosis of a failed system can be a complex process. Most vehicle shop manuals have a multi-page diagnostic flow chart that the dealer mechanics use to solve failures. If there isn't an obvious problem like a broken wire, a blown fuse or a leaking vacuum line then the problem most likely lies in the brains of the unit or in the switch that sets the speed and contains the other functions of resume and accelerate. Most cruise control switches are on the directional signal stem, a multifunction switch assembly with fine wires that break due to the constant motion of the wires as you use the directionals in your daily travels.

speed sensor