Dimension 2300 random reboots

Hello,

I have a Dell Dimension 2300 that I replaced the motherboard in two months ago after the orginial motherboard died. It has been sitting here collecting more dust than anything. Well initially I put XP Pro SP2 on it and it worked fine, how ever it would periodically reboot instantly for no reason. For example I could be watching a video and then the screen would go black, the numlock and other lock keys on the keyboard would flash indicating the PC was in boot mode. Then it would go through the reboot process. No bluescreen, no memory dump, nothing. The really strange thing is that sometimes it'll go a week without a reboot, then like this evening, it would even reboot while it's booting.

I tried numerous things to troubleshoot. Installed different OS's but that didn't solve anything. Removed all PCI adapters, disconnected all ide devices (except the hdd), flashed down to the A01 bios, turned of unneeded bios settings like USB, etc. Nothing, it still randomly reboots. I even stuck in MemTest x86 to check the memory and it even rebooted during that time. The only thing I can remotely think of causing this at this point is either the powersupply, memory, or motherboard. Are there any ideas for this? Powersupplies run $35-$45 on eBay and while I would gladly get a needed one, but get one and find out that isn't it, would be money down the drain.

Ideas?

Brad