For the past long while (months at least) my PC has been shutting down at random. Can anyone think of a reason why?

At first it would shut down (instantly turning off, like when you press and hold the power button) and then start back up again after a few seconds.

Lately, though, it s been shutting down and then not starting back up again. And I ve noticed that when I do turn it back on the BIOS settings are lost and I have to either load defaults or set them myself again. Once past that it runs fine again (until, of course, the next time it shuts down randomly). The system time also gets messed up but not the date. In case that s relevant.

My system:

i7 920 (stock clock)

Sabertooth X58 MoBo

6GB DDR3

GTX 460

Antec CP-1000 PSU (CPX form factor)

Antec Dark Fleet DF85 case

The PC is quite old (at least 4 years). CPU used to be overclocked but then the ambient temp shot up like crazy during summer and I couldn t keep temperatures in check so I decided to just settle for stock. Case had one fan break at the top (something fell on it...). I removed the fan.