Bump for relevance.
I have this exact freaking problem. Almost the same system specs, too. And I think it caused a video card to die on me as well, not to mention the lost system time trying to diagnose it.
Athlon XP 2500+, Radeon 9800 Pro, MSI KT6V-LSR (VIA KT600 chipset), two WD hd's, two optical drives, 2x 512MB Simpletech Nitro ram, Antec Smart Blue 350W. Power supply looks fine, always did. My problem started when I stiched to an MSI motherboard. Plus RPM sensor is dead, thinks my CPU speed is 0 intermittently, for extended periods of time.
Ram's new, so is the CPU, and both are top-of-the-line, lifetime-warranty things, and the PS isn't shabby either. Plus the error happened in a CD booting minimal Linux kernel, meaning it wasn't Windows. And between my PS at total idle, my new CPU, or Simpletech's Premium memory, I say the mobo's at fault. I don't want to loose a $300 video card to a $50 mobo.