Check the PSU. I've had some problems with cheap crappy PSUs either 300-500W. The problem was that too much devices, while booting, like CDs, HDDs and all, requires more power (not Watts but Amper) so the crappy PSU cannot deliver it so it restarts it crashes. Sometimes you can hear KLANG when the HDD takes too much power and crashes...
I couldn't believe how much I gained with hi-quality PSU. With my old computer I was able to OC my celeron 466 to max 580MHz, but with quality PSU i was able to OC it to 666 Mhz
Try another PSU.