Hope you have had some success with this.

I agree to some extent with Und3ertak3r, but in my case it was definitely a hardware problem with the sound card (pretty unusual), which proves this can happen .....

Switching back to the on board sound is a good idea to eliminate this possibility - but as I had an old PCI sound card, I thought that would be a slightly better test, as it would eliminate the chance that there was a problem with the PCI slot it was using. I was rather surprised that the company I purchased it from replaced it without any questions, and even offered me a free refund instead. On the other hand I did give them chapter & verse about what I had tried to fix it.

You should be OK on the cooling front - your P4 is obviously OK (mine runs at 45C in really hot conditions - no problems), but what I meant about the cooling is that the heat has to go somewhere.
So if your graphics card is next to the P4 cooling fan then it will get affected, unless you have decent cooling in your case that can keep everything at an acceptable temperature.

Could be a hardware/software video card problem, but I think that is highly unlikely, as normally this would fail fairly quickly. On the other hand a RAM problem can cause this type of thing to happen, but you would also expect random crashes when running something other than games.