Spanners -
Just in case it's an intermittent hardware problem, here's a trick I used many many moons ago.
You used to be able to get an aerosol can of freezer spray (I guess that it's just raw freon). You squirt at the hardware to see whether you can get the PC to freeze (unintentional pun). The theory is that as the box (and contents) warm up, you get some thermal expansion and this can cause intermittents (hairline cracks in PCDB tracks, dry soldered joints etc). Freezing it with the spray isolates where the problem is.
Might be worth a try.
Good luck!
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Feline
(I Sit, I Watch, I Smile)