I have a 97 grand prix gt and when i rev on it and just let the rpms come back down they drop lower then normal but come back to normal it goes about 200 to 500 rpms below normal depending on how high i rev it and also going down the road sometimes and when i lift off the gas the rpms drop down but then pick right up iv changed the*idle*air*control*valve
*and it runs good and idles fine it has plenty of power and the throttle responce is good and it only acts up every now and then it doesnt do it all the time what can be causen this