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Could it possibly be a faulty motherboard? Is it a desktop or a laptop? If it's a desktop and it's more than a few years old, check for leaky capacitors. There was a big issue a few years back when motherboard vendors used substandard capacitors. It might possibly be a source of your problems.
Have you also tried disconnecting any un-needed hardware and only booting up with the basics (like a single stick of RAM, only one HD and only CPU and fan connected? Is the PC overclocked? If so, drop the settings back down to stock or failsafe mode to test.
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