Car Shudders When Shifting

I have a Stock 98 GP GT. Sometimes when I go WOT or close to WOT... the front end/dash shudder violently for a split second right when it shifts 1-2. It also happens, but not as bad on the 2-3 shift. This happens abot 75% of the time that I go WOT. Can anybody tell me what's going on and how to fix it? Thanks.
-Will

I have a 98 GTP and it does kinda the same thing. Between first and second it "jerks" and then maybe just a little between 2-3 but only after the engine gets warm. When I first start going when the engine is cold it doesn't do it. It's just a quick little thrust between 1st and second gear. I have plenty of oil and transmission fluid. I wish I knew the answer. If anyone knows please help!

my 02 did something like that was the valve solinoid, the valves where sticking the dealership said.

My GTP will do it with the performance shift off. But not when it's on. Dealer told me the trans needs to be rebuilt. Ya, right.
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP002.jpg