It's quite possible that it's caused by a dislodged memory stick, which are really easy to bump out of their sockets accidentally. I recommend removing all but one, and adding them back in one by one, testing the boot process each time to eliminate the possibility of a faulty stick.

Also, I'd say it's a good idea to check the temperature of the CPU in your BIOS right after a power-off, since overheating can cause this problem sometimes.