I came across this 29/5/08 on a system I had originally built and supplied in Jan 2005 and fitted an upgrade AGP graphics card in 2007.
This is a pain in the butt as you have no clue as to what the problem is - I eventually traced it to the ASUS graphics card which was getting too hot.
Fitting a botch job fan to it cured it for about an hour when it all fell over again: reboot with a new cheapo AGP card with a fan on it but no improvement.
There comes a time when the messing about has to stop as it ceases to be profitable - even on cheap time in the middle of night when nursing a sick dog so I made an executive decision and chucked the board, the CPU and the memory into the bin. Rebuild and off itwent like Tigger* on speed.
Pity is that of the 3 components that were thrown away only one was faulty but it was not worth finding out. A condemnation on our society and the way we discard good kit because it is too expensive to mess with it.
End result I think will be client with a machine that goes like the clappers which he does not need.
Ah me; oy veh.
*Tigger - kangaroo type character from A A Milne's Winnie the Pooh - see
Tigger from Winnie the Pooh | Winnie the Pooh