Laptop repair 201 (or 301) ...

Sigh my "Go to" laptop has developed a new problem. It is randomly rebooting. Dell D630 running XP about 4 years old. Yes, I know I should bite the bullet and upgrade probably to Win7. But, I like the D630 and I like XP. Recently L convinced me to bite the bullet and replace the screen which would flake out under heat. I did that and it was fine for a while. Now I have random reboots. BIOS diagnostics are clean. Norton says no viruses. Malwarebytes Anti-Spyware says no problems. Here is the kicker: IT DOESN'T HAPPEN IN SAFE MODE. Does the Nvidia card (which has known problems) run differently in safe mode? Is that driver not used and the default on-board video used in that case instead or something?