It's here....

[JustAGuy]

In my experience, random lock-ups not attributed to driver failure, hard drive or video failure means motherboard failure or CPU fan failure + CPU overheat.

I doubt it's driver or video related, since you already swapped out that stuff. Your hard drive is fresh and clean, so that leaves the motherboard or CPU problems.

If these seemingly random (they're not, because they're caused by something failing) lockups appear only after the machine has been running for a while, and then once you reboot, every few seconds/minutes afterward, then it sounds like a CPU fan has failed, and your processor is getting toasty. But if you're sure the CPU fan is fine, then you might consider the motherboard. Something on the motherboard might have pooped (that's a technical term) and it might need replacing.

It's the worst thing to replace too.

So, I suggest you run it with the case open, and at the first lockup, check the state of your CPU fan, and any other fans on the motherboard. Give it a whiff -- if you smell burning silicon, your problem is closer to being solved.

If it turns out not to be hardware related at all, and say you have a huge memory fault causing error in your OpenGL app, then, well, that's all you. :)