You've covered the biggies - power and heat.
Is it happening before the OS starts to load?
I'm wondering if it may be bad memory - can you boot into DOS and run memtest86?
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"The problem with any ideology is that it gives the answer before you look at the evidence."
—Bill Clinton
"The greatest obstacle to discovery is not ignorance--it is the illusion of knowledge."
—Daniel J. Boorstin, historian, former Librarian of Congress
"But the Senator, while insisting he was not intoxicated, could not explain his nudity"
—WKRP in Cincinatti
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