Hardware Fault?

I've run into a small problem...it used to be small anyhow. I'm using winXP home, AMD Athlon 2000+, a Leadtek 8xagp Ti4200 128mb ram, sblive! platinum, GA-7VA motherboard, 512mb DDR (2100) ram and a 400w power supply. Sometimes my computer just reboots for no reason, I check the temperatures and everything seems fine. I'll be in the middle of a game and it will just reboot, even doing music...it'll just reboot. I can't remember having this problem with win98SE but then again I never used this PSU and motherboard and vidcard with 98. I used to use a 4xAGP Leadtek G4mx420, and still the same problem so I know its not the videocard, I've even tried uninstalling the drivers and trying different versions. I've upgraded and downgraded drivers for almost absolutely EVERYTHING but still no luck. I'm going to put this in the hardware community also, just in case its a hardware fault.

Do any of you have any ideas? if theres any specs I've forgotten please let me know.