Bios flashed to the latest revision, adequate cooling, correct jumper settings and all that good stuff?
Try conservative bios settings (Set everything to the "slow" or "most compatible" setting, some bioses call it loading Fail-Safe defaults)
Everything all connected nice and tight?
there's nothing obstructing the cooling of the system, is there? like dust-clogged or even inoperable
fans, excessive wiring (especially IDE and floppy cables)?
Try running with your case open, if you aren't already, because if your case is poorly designed (e.g. Style over solid construction and good airflow efficiency), it could make a huge difference.