Well, that sounds like a heat issue, and letting it cool down completely allows you back in, until it heats up again. You might try opening the side panel and make sure it is free of dust and dirt. Be sure to touch bare metal of the case before reaching in to discharge any static in your body.
While in there, Inspect the motherboard for bulging or leaking electrolytic capacitors. These failed or failing capacitors are a common cause of sudden, but seemingly random system lock ups and reboots. The capacitors look like tall soda cans, many of which surround the CPU socket.
All older motherboards, and many of today's less expensive motherboards use electrolytic capacitors containing a
liquid electrolyte. Failing (including flawed and/or abused/over-heated) capacitors literally bulge at the seams due to excessive internal pressures. Extreme (and very rare) cases result in a firecracker type explosion that can really stink up a room. Typically, electrolyte just
oozes from the pressure relief points stamped in the tops of the capacitor casings (seen as a symbol or letter). The electrolyte can be caustic to motherboards and flesh. Look for white to dark-brown, dried liquid or foam on the tops or bottoms of the capacitors. Bulging capacitors are a sign leakage is about to occur.
A motherboard with bulging or leaky capacitors can be repaired, but often it is more cost effective in the long run to replace the motherboard.
Be sure to
first power down, unplug the computer, and keep yourself discharged by touching the bare metal of the case.
Leaky Caps:
Bulging Caps:
If all looks good, blast a desk fan into the interior, boot up and see if it holds. Peek inside and make sure all your case fans are turning too.
This could also be caused by a weak or failing power supply, or bad memory. You can test your RAM using one of the following programs. Both require you to create and boot to a bootable floppy disk or CD to run the diagnostics. Using the floppy method is generally easier and yet another reason I still include floppy drives in my new PC builds. However, the CD method is just as effective at detecting RAM problems. Allow the diagnostics to run for several passes or even overnight. You should have
no reported errors.
Windows Memory Diagnostic - see the easy to follow instructions under Quick Start Information.
or
MemTest86+ (for more advanced users) - an excellent how-to guide is available here.