Car bucking at speeds above 60 mph

Take extra caution while performing dirve cycles on public roadways. Drive cycles can be difficult to follow under normal driving conditions. Begin by: - Start the engine. Idle the engine in drive for two and a half minutes with the A/C and rear defroster on. - Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle. - Hold at a steady speed of 55 mph for three minutes. - Decelerate (coast down) to 20 mph without braking or depressing the clutch. - Accelerate back to 55 to 60 mph at ¾ throttle. - Hold at a steady speed of 55 to 60 mph for five minutes. - Decelerate (coast down) to a stop without braking. If all other emissions systems are functioning properly, completing the drive cycle above will set the "readiness flags" required to pass the smog check. It will not hurt to perform the drive cycle 2 or 3 times before the smog test.

Try an owners forum before you call the dealer. My guess is there's a speed control unit problem.

My 99 GMC savana 1500 was chugging at like 50 60 mph and gas gauge was stuttering ..it turned out to be computer..I had it re flashed by GMC for 150 bucks ..that fixed all...hope this helps

could possibly be your head gasket or even a cracked head

Here is the info

NOTE: The ignition key must not be on prior to the cold start otherwise the heated oxygen sensor diagnostic may not run.

1. As soon as the engine starts, idle the engine in drive for two and a half minutes with the A/C and rear defrost on. OBDII checks oxygen sensor heater circuits, air pump and EVAP purge.

2. Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle. OBDII checks for ignition misfire, fuel trim and canister purge.

3. Hold at a steady state speed of 55 mph for three minutes.

OBDII monitors EGR, air pump, O2 sensors and canister purge.

4. Decelerate (coast down) to 20 mph without braking or depressing the clutch. OBDII checks EGR and purge functions.

5. Accelerate back to 55 to 60 mph at half throttle. OBDII checks misfire, fuel trim and purge again.

6. Hold at a steady speed of 55 to 60 mph for five minutes.

OBDII monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors and purge functions.

7. Decelerate (coast down) to a stop without braking. OBDII makes a final check of EGR and canister purge.

U have to operate the engine at all kinds of diff speeds and drive modes, it is called a OBDII drive cycle, here is how u do one.
1. As soon as the engine starts, idle the engine in drive for two and a half minutes with the A/C and rear defrost on. OBDII checks oxygen sensor heater circuits, air pump and EVAP purge.

2. Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle. OBDII checks for ignition misfire, fuel trim and canister purge.

3. Hold at a steady state speed of 55 mph for three minutes.

OBDII monitors EGR, air pump, O2 sensors and canister purge.

4. Decelerate (coast down) to 20 mph without braking or depressing the clutch. OBDII checks EGR and purge functions.

5. Accelerate back to 55 to 60 mph at half throttle. OBDII checks misfire, fuel trim and purge again.

6. Hold at a steady speed of 55 to 60 mph for five minutes.

OBDII monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors and purge functions.

7. Decelerate (coast down) to a stop without braking. OBDII makes a final check of EGR and canister purge

I would really have that exhaust checked again, as well as have mechanic put fuel pressure gauge on & drive it up to that speed and see what happens fuel pressure wise. With exhaust, it's not only cat converter that could be causing reduced flow.

Sounds like the EGR valve is giving you some trouble. If it is happening during steady cruises, That is where I would start. Do you have a check engine light on?

Computer firmware needs upgrade

Take your car to a Chrysler dealer shop and have them update the transmission program.As your tranny wears it loses it shift point and stutters between 40 - 45 mph looking for the right gear to change to..Had same problem in 94 Concorde and 94 T&C minivan.Reprogramming tranny fixed both cars.