Over the past couple of months I've been having the occasional problem with my computer.  At completely random times, I've been getting BSOD's with a "Machine Check Exception" error message.

I've checked the Microsoft Knowledgebase for assistance.  But their suggestions don't really seem to apply.  I'm not overclocking my system at all.  I've checked the power supply with a testing device and there doesn't appear to be any problems with it.  I've also run the memtest program and can't find any problems with the memory.

Is there anything else I can do to try to diagnose the exact cause of the problem?  I'd rather not replace the mainboard or processor without knowing exactly what is causing the problem.

System specs are as follows:

Windows XP SP2
ASUS A8N-SLI Premium
Athlon64 3200+
1GB RAM (2x512mb Geil PC3200 DDR)
GeForce 6800GS 256mb