thanks in advanced for any help - this is driving me insane!!!

ok, first off i have recently upgraded my computer (mainly for gaming) I changed mobo, got a 6600GT, and 1024 DDR400 Ram

basically the problem is that my computer slows down from a silky 60fps to like 3fps when playing games. The weird thing is that it usually takes 30 or 40 minutes for this to happen...

I've tried everything within my knowledge to fix this - windows update, vid drivers, virus scan etc. at first i thought it could be a heat problem causing thermal throttle on me P4, however the temperatures seem very reasonable...

pleeeeeez help...