Problems while accelerating/driving
 

I have a 2001 S-80 t-6, while on a drive to Milwaukee this weekend the car started acting up and I have no clue as to the cause.

here's the run up..

Two weeks ago i noticed that at speeds of 70-80 the car would shudder randomly. Not very noticeable, but i could feel it in the pedal and the movement of the car.

I took it to the local shop and replaced the tranny filter and fluid. Car drove fine for a few days after that, but now the problem is significantly worse.

Whether in cruise control or with the foot on the gas, it feels like the brakes are being applied or pumped. it happens about once every 10 seconds and it's enough to make the front end of the car dip down, but doesn't effect the overall speed, car just keeps going.

I don't think it has anything to do with shift points as i was doing 80 mph and by then i should be in the highest gear. To test this i dropped it into manual mode and still had the same issues in 4th and 3rd gear.

To me it feels like a vaccuum line broke or is not working correctly somewhere.

Anyone else have an idea, or better yet, the answer?

Is the ABS warning light lit?

No, no new engine codes either. Just one bad O2 Sensor that's been bad since I bought the car.

I had the ABS rebuilt when I bought the car.

I do have a feeling it has something to do with the traction control messing with the ABS. Maybe not noticeable enough for the light to be turned on.

My $200 http://www.ppc-diagnostic.com tool has paid for itself already. An intermittent error can set a code that alters the behavior of the module, so being able to clear them without paying the stealership $100 every time is great.

Um, that's cool, but if no codes exist, the scan tool does no good.