I used to have a 2005 Grand Prix GXP that was the 5.3L V8 engine. It was the worst car I ever had as far as reliability was concerned. When I bought it (used) I was not disclosed that it had 4 transmission rebuilds. After I bought it, I started having problems... The issue was that when I would come to a stop at a traffic light or stop sign, I would lift my foot off the brake and proceed to press the gas pedal and the RPM's would shoot up but the car was not moving forward... randomly it would eventually just go THUNK and kick it into gear and cause the car to jolt forward... once moving, the car and transmission ran fine. This happened often enough that I eventually got fed up and asked my dealership to look into the problem. Of course, when the technician took the car out for a drive, he never found any issues in the way the car performed. He handed me small ODB connector that was attached to a little recorder that monitored my cars activity.
Then one morning I was exiting my underground parking, I had to drive up a tall angled ramp area to get onto the ground level.... when I accelerated up the ramp, same thing, revs up.. no go.. then CLUNK the wheels start spinning and off I go. The recorder revealed absolutely no information at all.
Eventually I had to give in and have my car go in for service for the who know how many'th time. It eventually had 2 seperate transmission rebuilds, and everytime it would be great for a few hours of driving, and then it would do it again. After a total of 6 transmission rebuilds, and no help from GM, I decided to say **** it and sell the car. Never been happier.
The thing that always bothered me was that no one had any idea or information on why these things happen. Not the master mechanic, not GM, no one. They just told me it was probably the "Clutch" or "Torque Converter" int the transmission but again, after 6 rebuilds, why would this happen? I have never been able to get a straight answer... if it wasn't for those problems, I really liked the car but it was too much hassle. Any ideas?