[4Tran]

This is one of those problems that are just a pain in the neck to isolate. My first assumption is that it's a hardware problem. Make sure that, at the very least, your mainboard is updated to the latest BIOS, and that you try a couple of different (older) video card drivers. You should also reset your mainboard CMOS settings to optimal or the safe mode.

If you have the time and patience, make sure that your problem doesn't originate with your software. To do that, you'll have to do a fresh install, load with just bare drivers and minimal software. You'll want to make sure that you have the mainboard chipset and IRST drivers installed. If the problem still manifests, check what you were doing when it happened - was the system running a game or some other heavy load, or was it just doing normal tasks like playing a movie or browsing the internet?

If you want to skip that step, I think that the likely causes of the freezing are, in order:

Mainboard
Power supply
RAM
Video card
Hard drive
.
.
.
CPU

Unfortunately, there's no reliable way to test a lot of the quirky mainboard problems other than to update the BIOS and hope that that fixes the issue. Otherwise, the simplest thing to do (if you have the parts) is to try out your system with a different mainboard.

The same goes for most of the other parts. The exceptions are the RAM and hard drives. If you've ran Memtest, then it's safe to assume that it's not the cause of your problem. You can find testing utilities from your hard drive manufacturer(s) and test those as well. If you're not sure about what you're doing, you might want to take your computer into a computer repair shop - make sure to ask about prices first though.