Quick way to make sure it is or isn't an OS issue would be to download and burn a Live CD of some Linux distro. Ubuntu is easy. Boot from that. If it still locks up, it's hardware.
Sounds like a hardware issue, chip locking up or shutting down due to thermal problem. This would be the CPU, GPU or North Bridge. Start with the CPU. Take off the chip cooler, clean the thermal compound off with isopropyl alcohol and cotton swabs. Apply some new quality thermal compound. A little dab on the chip is what I usually use, then dab it around with my fingertip until it's a thin even layer. Less is more in this case. The purpose is to fill any tiny voids between the metal surfaces. Too much and it forms an insulator. Some companies put on so much it looks like a birthday cake exploded in there. Test it out. If that didn't solve it, replace thermal compound on the other components that have coolers.