how to debug

[rray_1 at comcast.net]

I know this is all very vague but I have to start somewhere
I have a PIII pc that served me very well for several years
A month or so ago it started rebooting any time I ran X
I presently have CentOS 5.3 installed
I have tried Ubuntu and Fedora with the same results
X starts and will run normally for about 1 to 10 minutes then reboot
I have a Nvidia AGP card and a ATI PCI card both produce the same results
If I start X on the machine and only access it remotely via ssh it runs 
without any problems
I have been using the xorg packages that come with the distributions
Presently 1.1.1-48.52.el5
Where do I start determining the problem

Thanks
Richard Ray