Start by running through the steps to this thread
. Note any changes to the behavior.
Next, go to Step 10 of this guide
and run the recommended programs. Focus on Prime95 with Real Temp and MemTest86+. MemTest will need to be burned to CD or put on a flash drive to work, and should be run overnight. Come back with the results.
Chance are that it is a RAM problem, but there are many possiblities for the culprit right now.