My guess is something's wonky with your PSU. I've known the strangest things to happen then. And it doesn't matter if the PSU is brand new or not. If you can, try and swap it with one that is old and trustworthy, see if that changes anything.
Sorry to bump this old thread. I finally got around to fixing the computer last weekend (lack of free time really sucks) and it did turn out to be a bum PSU.
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