I have had a similar problem with my 99 GTP it was about 2 weeks after I bought it I was driving along after about 2 hours and it started bucking and shifting hard check engine light came on I pulled over shut the car off then decided I go another 3 hours to drive and started up again and the problem seemed to be gone check engine light still on mind you so I put a scanner on it as soon as I could and found that the torque converter clutch solenoid was sticking the code being P0741 it hasn't done it since then and its been about 5 months but every once and awhile it shifts hard... the trans fluid seems fine it might be a little low but I am going to add an additive sometime and see how that works the only problem is it doesn't do it all the time so its hard for me to diagnose it
