Maybe this will help someone. I have a 97 GTP with 180K miles on it. I just purchased the car not too long ago. I know the supercharger on it came from a junkyard and the current torque converter is brand new, and that is about it.
The car was running fine until I downshifted to pass a semi on the highway. When it shifted back into overdrive is when the problem started. It felt like a tranny shudder to me.
I asked my personal pontiac mechanic what the problem could be and he said that the trannys will shift funny if there is an engine miss. Hooked up the scantron to the OBD-II port and behold - multiple cylinder misfires.
Next step. Three new coils. New wires and plugs. And, the miss was still there. The next thought was bad gas ( I didn't think so, but I had to indulge my personal mechanic.) So I put 93 octane in with injector cleaner and octane booster and took the car for a nice long ride. That was saturday night. Sunday morning - no transmission.
When its cold it will move a little, but once it warms up there are no gears. I have a junkyard tranny to put in it and that's where I'm at.
I hope this might help anyone who needs it. Good Luck.