My STS when cruising between 40 to 45 MPH and at higher speeds it appears that the engine misfires. The car feels like it is riding over a washboard. I have used fuel injector cleaners and no change in performance. The car runs smooth at idle and at full power accelatron. My OnStar reports everything is normal. Thanks for you help.

Joe

 

sooooo one problem or two?? for misfire, for sure change plugs and wires.
how many miles on the car??
and riding over a washboard.....????? is that from the misfire??
is the trans okay?? check trans fluid. run car out, from stop half throttle and feel shift. is the car shifting properly??
hope it helps
rickyd