It sounds like it could be one of two things. I have heard some people that have had trouble with a speed sensor switch of some sort that causes the tranny to not shift right or erratically. They thought transmission was no good, but found out the only problem was that switch/sensor. The other possibility I can think of is that it is overheating. I have heard that these transmissions have a tendency to overheat under real aggressive driving. Also it may be bacause your cooler line or filter is plugged up and you need a tranny flush. Have you had the transmission services done on the car 30,000/60,000/etc.? Mine is coming up on 30,000 so I already have a list of maintenance I need to do. Let me know if you get anything. Hope this helps.
