My 2003 pontiac grand prix gt. 3800 transmission tends to start slipping bad at times, it stops pulling, it fails to change gears, the speedometer goes crazy and tends to just stop regestering at time

I had a mechanic change out the Crankshaft Position Sensor on my 2003 Grand Prix GTP that solved several problems. I got the idea from sylvester2000 but he wasn't the only guy on this site who recommended it. The car would stall every once in awhile but wasn't throwing any codes. Usually stalled at idle at first, then in stop and go traffic or when coasting to a red light, then with the air-conditioner on, then if I made a left turn (believe it or don't). It would run fine for 2 or 3 weeks and then just quit. It over-reved one time before it died. The GM dealership couldn't figure it out because all their tests showed no problem and of course it wouldn't die on them when they test drove it. The part is fairly cheap but the labor was spendy - difficult and time consuming. $360 total. I'm not a mechanic so I found someone who would do it just on my guess and insistence. The car runs real good now. No stalls and more power. Sweet.