patio and carbon have both got a good point.
Overheating will definately cause you're computer to shutdown randomly.
However, just out of curiosity, you didn't change your processor voltage through bios setup did you?
overclocking if you don't know what you're doing can be highly dangerous, and can fry you're motherboard, if you're not careful.
If your computer is overheating, then i suggest you buy a new fan.