XP Pro crashilng and restarting

[travistee]

I have an Asus p5p800se XPpro SP2 Pentium D2

It started to crash and restart.
It goes to a black screen and then restarts.
It works for a while then crahes again.

I tried going to a clean boot, uninstalled CA antivirus ans spyware, plugged it into the non battery backup plugs on the UPC.
The PSU was replaced about six months ago.
All the temperatures and fan speeds on Asus probe seem ok.

Any ideas on where to start to look for the cause.