I recently finished putting together a new computer. When it's working, it runs fine. However, at completely random times, it will power off and power back on very quickly, with no error message or anything. The temperatures are all under 50C, and usually under 40. It can restart during an extremely CPU/graphics intensive application, or when I'm simply browsing the web, or even when the system is idle. I am running Windows XP home edition with SP2.

Specs:
AMD 64 3500+
DFI ultra-DR
X850 XT
160gb 7200rpm 8mb buffer Samsung HD
dvd/cd combo drive
Aerocool Aeroengine case
500W Aspire power supply
1gb (2x 512) pc3200 Corsair value select


Any help is welcome!