My 99 GXE,with auto transmission, has 40k on it now. It has been smooth all the time. But recently while driving at 40mph, I feel I have to press the gas pedal harder to keep the speed or accelerate. And it seems like the transmission doesn't know which gear it should stay. It shifts up and down, and shakes the car very hard.
Does anyone know why is that?
Really appreciate your help.

 

Sounds like your coil packs maybe starting to go. Samething happened with our 99, esp. around 40, when you put foot on gas, misfires. I replaced one coil pack which test0poor only to find another one 30 days later. Now after another month, a third coil pack is causing misfires again. I just ordered 4 new coil packs. I now can see why the dealers go ahead and replace all 6 at one time now on these 99.'s