GeForce 6600GT - Full screen = lock up?

Just this past week I upgraded my video card to a GeForce 6600 GT. Ever since any games I play lock up a few minutes after starting them up. After doing some research online it seems this kind of card has been having issues with locking up in games. I was told by many different people that my issue was my PSU. So I upgraded to a 600 watt (the card needed atleast 350 and I had a 250). And I still had the problem.

After alot of stress and pain and trying everything I could. I came to relaise the only time the game locks up is when it is in full screen. If I play a game in window mode, it runs perfect. Now I have no problem playing games in a window, however not all my games offer a way to change to that mode. So I have a few games I am unable to play.

Does this make any sense to anyone? I just don't understand how when I play in windowed mode I have NO issues.. the second I turn to full screen it freezes. I hope someone has an idea to what this issue is.

Thanks for your help in advanced!