Computer freezes in games, need help

My system is:

mobo: Asus p5k pro
cpu: Intel 8200
cooler: Coolermaster hyper tx2
ram: Kingston hyperx 1066 Mhz, 2Gb
gpu: Asus 9600gt 512 mb
HD's: 3 x WD 500 gb aaks, 2 x 250gb jb
DVD's: LG sata dvd r + Lg sat dvd
psu: OCZ game xtreme 600w
OS: Windows xp proffesional, SP2

Like 2 min after running almost any game I get a display freeze(green dots all over the screen if that helps) along with sound repeat. This happens on mark3d, assasin's creed, heroes of might and magic, bioshock, wow etc etc but strangely enough not in warcraft III when I play dota. It even freezes when I play pkr (a poker client that is 3d).

So, Ive run memtest and prime for several hours. No errors. The psu is new (I originally bought the ocz stealth xtreme 600w but it was defective and I got the game xtreme), so its probably good. The gpu works at around 47c at windows so thats fine I think. The mobo works at 50c which is a little bit high, but I searched at the asus forum and others said the same mobo runs that high. The cpu also runs a bit high, 52-55c but Im guessing thats not the problem.

So really the only thing I thought maybe it was nvidia drivers problem, I re-installed those and installed the last directx, but still having the same problem.

So really Im in no man's land, if anyone has any ideas that I didnt think off, be my guest, any help would be appreciated ....

Download ATI Tools (http://www.techpowerup.com/atitool/). Yes it does work with nvidia and scan for artifacts, maybe thats your problem (artifacts are usually caused by faulty video RAM, even at low temp it can happen)

i head a similar problem(freezing in games,sound loops then all ok) some time ago with an asus motherboard(all the temps ok). (I tested the psu, ram and video card at a friend and they were fine).
The problem was with the on-board sound: a sound card will fix it or go to :
run-type dxdiag-sound tab and reduce the level of acceleration.