My first thought when something like this happens is to power down completely and check that the memory cards and processor are still firmly in place, not coming out of their sockets. They expand as they get warm after the PC is turned on, contract after it's turned off, over time this can loosen the clips, pins, or whatever holds them into the PC. A poor contact is the result, and that leads to freezing.
One other rather strange possibility - for various arcane reasons the chip that controls the keyboard has a lot of extra functions in PCs, and a failing keyboard can bring down the whole computer. It's not likely, but it can happen.
