honestly, heat could very well be the problem, or mobo. I've had a computer do this before, and I'm still waiting to fix the issue. starting with new CPU. I wish they made mobo testers, or something that (outside of the actual computer) tested the hardware. either way, it looks like you have crossed the bases, consider the mobo tho, that's the last thing I can think of (short of virus and I wouldn't suggest it as you have done your homework very thoroughly)
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Current system: Shuttle Running 1.23GHz Intel Celeron, 256 ram, XP Home 120GB hdd
Building new system w/ EVGA 680i mobo, 2 gig Mushkin 4-4-4-12 RAM Intel q6600, EVGA 9800GTX, sitting in an XClio Windtunnel
