Speed sensor volvo s70 1998. Is this an easy fix?

I will assume you've tested ALL the breakers with a VOM...

There is no "easy fix" as only 2 of the 4 codes are for the same thing...for the P0136 and P0141, its probably a bad Bank 1 sensor 2 oxygen sensor..the P0306 means cylinder 6 is misfiring (it could be bad spark plug, bad coil, bad injector, carbon on valves, low compression) and P1181 is showing a fuel delivery issue high pressure...as for the vibration, have all tires checked for balance and hum on passenger side is possibly a bad wheel bearing...and and all is best to have checked by your mechanic.

Since it works, even part time.The ECM and the cruise module are most likely good. Start at the cruise module plug. Pull it and look for corrosion first. Battery cleaner cleans this off. After cleaning it,use brake clean to get rid of the battery cleaner, let dry or blow it out with air. A lot of times this will fix the problem.

Could be a bad (VSS) Vehicle Speed Sensor. Check it its located on your transmission. Good luck

Several suggestions, some cheap, others not. The overdrive relay is the first suspect. It's located on the relay rack behind the fuse panel (usually). Cheap fix. About $10 on Ebay. Just be sure the part numbers match. I always carry a spare. The speed sensor might be another source of problems. It measures RPMs and feeds the info to the computer. Not an easy do-it-yourself fix. Also, the transmission control module. An easy swap with a spare from Ebay. It hides under the dashboard, passenger side. A solenoid on the transmission locks the OD in and out. Not an easy do-it-yourself fix. Volvos are notorious for flaky issues like this, so I would try all the simple fixes before bringing it to your mechanic.

99% Bad ABS Module.

Unless you have a cut ABS wheel sensor.
I fix them my self.

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Have the idle air control valve checked, it might just need a cleaning, or a new one. Check all the vaccum hoses for leaks, all it takes is a pin hole to screw up the sensors.

Volvo Radiator, Thermostat and Sensors Your cooling system's temperature controls include all coolant temperature sensors, Volvo thermostat, Volvo radiator or expansion tank cap, cooling fan(s) and fan clutch (if equipped). These cooling system parts function primarily independent of the engine but control the engine either through cooling or by sending control signals to your Volvo's electronic systems.
The Volvo thermostat is a spring-loaded valve that opens and closes based on the temperature of the coolant flowing through it. A high temperature reading followed by a drop to normal temperature (or a continuously low temperature) is a common first sign of a sticking Volvo thermostat. However, many other conditions may cause these symptoms, so you need to know how to eliminate each possibility.
The Volvo radiator or expansion tank cap is also a spring-loaded valve reacting to system pressure. It serves to maintain proper system coolant level at predetermined pressures. It must always be replaced with an exact replacement cap with the same pressure setting. Never use other caps except for short-term emergencies!
A belt-driven fan blade for pulling air through the Volvo radiator is usually on the Volvo water pump pulley and should have a fan clutch to control it. The fan clutch allows the fan to turn with the belt at low engine speed and "free-wheel" at higher speeds. A bad fan clutch either doesn't allow the fan to spin at low speed (overheating in traffic) or doesn't allow it to free-wheel at high speed (potential overheating on highway or reduced gas mileage).
An electric fan can be either by itself (usually front-wheel drive) or auxiliary (used with a mechanical fan). Both types are controlled via a temperature sensor - in the Volvo radiator or upper Volvo radiator hose or on the Volvo thermostat or Volvo water pump housing. This sensor is usually an on/off type switch with a fixed temperature setting. (Some vehicles may have 2-3 settings for multi-speed fans.) This sensor is commonly called an "auxilliary fan switch".
Other common temperature sensors are: 1) gauge sender (variable output); 2) warning light sender (on/off type); 3) lambda and/or fuel injection sensor(s) (variable to control fuel injection settings); 4) thermo-time switch (cold start valve control). Your Volvo may have other sensors as well.
Temperature control is critical to both performance and emission control. Unfortunately, this system is the most difficult to troubleshoot without proper equipment and diagrams. It's even more difficult with computers that adjust timing, idle speed, vacuum and fuel delivery automatically to make up for potentially faulty temperature sensor signals.
Maintenance of your cooling system sensors is virtually impossible since there's nothing really to "maintain". Keeping them clean both internally (coolant replacement) and externally (engine cleaning) is the best way to ensure trouble-free driving. Checking and replacing all parts at the factory-recommended time or mileage limits helps as well

Was it the "Idle air control valve" you replaced? If not, I would.

The code is for the Vehicle Speed Signal (VSS). The VSS is produced by the ABS electronic control unit by averaging wheel speed sensor inputs and sending a digital signal via CAN bus to other modules that use the VSS. To test the ABS, a Volvo-compatible scan tool is necessary. If one is not available, the car should be sent to a Volvo dealer for ABS diagnosis. I can tell you however that most of the cars are fixed by replacing the ABS Control Module.