Hi, i would say you need to prove if it is a problem with your XP install or a hardware fault.
First boot from the XP CD and do a repair install.
This will overwrite any corrupt files and return XP to Service pack 1. Run like this for a while and see if the problem has gone, if not start looking at PSU/Memory/motherboard/overheating.
If it runs ok try Service pack 2 again with the SP2fix and see what happens.