Dell PC rebooting randomly - unsure of cause

We have a Dell Optiplex 745 that reboots randomly when we use it.  It is running Windows XP and I have reinstalled Windows XP on this machine manually numerous times to see if that was the problem.  There is no BSOD or any kind of message.  The screen will freeze, go blank, then reboot to the BIOS screen and continue loading Windows.  This problem has been going on for several months now and it is very random and intermittent so it is very hard to reproduce. Dell has already replaced the motherboard and power supply, but the problem continues.  Dell recommended we unplug the network cable and let it sit for a week, which we did.  During that time it never rebooted so now Dell thinks it is not a hardware issue but a problem with our network/domain.  However, we are very sure it is not our network or domain or else the other PCs on our network would be experiencing the same problem, especially those PCs that are of the same model and hardware configuration.  The PC passes all of Dell's diagnostic tests and I want to think the problem may be the CPU, RAM, NIC or HD.

Has anyone else ever encountered this?  Any suggestions? Please advise.