Asus Moboard - P4 3Ghz Shuts down randomly

I have an
Asus P5P800 Moboard -
Intel P4 2.(?) ghz proc
2 gig RAM
Win2K (was running Ubuntu - got hooked on WoW

System had been humming along nicely - then occasionally would shut down on its own -

First thought was the power supply - so it has been replaced - ATX - giant fan blowing into enclosure, and external.

No change - system would sometimes get through to Windows logon screen - and hum along nicely - last day and a half - gets to login screen and dies.

Tried 'Safe Mode' - no change. Have a new heat sink on order and keeping fingers crossed.

Any other ideas?

 

You may want to go into the bios and check your cpu temperature.