In my experience, excessive heat usually manifests as BSODs...(though I wouldn't rule it out)
...random reboots were usually either from a failed component (memory, cpu, pci card) or voltage fluctuations from the PSU.
Are you/have you been running MBM so you can check for temp/voltage fluctuations that started recently?
What OS are you running? Does the task manager show any new, unusual apps running? Is your PC using more memory than usual? Could you list all components in your machine, including removable media that may be attached to add-in cards or USB ports?
What do the error logs say?
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Everything aircooled

(I honestly don't know why you guys let me stay a member...

)
Always folding:
WXP Pro:
KD7R + Palomino 1800+
W2K3:
4x P3 Xeon 500 512KB (4 clients)
Linux boxes:
none

Had to put them to use doing *real* work.