i have a 99 grand prix gt. it has 177000 miles on it. i got it about a year ago and the first time i drove it i noticed that going up hills or even on level roads when i hit the gas lightly the car starts doing a rocking - like motion and on the odometer the needle starts to bob in sync with the rocking. i took it back to the dealership and they "fixed it" but it started happening again a few months back. the odometer is normally at the 2 when it bobs. Anyone know what this could be?