Have you recently physically moved your computer?
maybe to swap a card over?
Reason why is because you could simply have created poor connection as the motherboard flexed as the case was moved, or a card inserted. Many pc stability problems can be fixed by reinserting the cards & memory.
Another possability is that your powersupply could be on the way out. I've heard alot of ppl have troubles after installing a new geforce 4 128mb card. They use alot of power, and can strain a system to breaking point.
You should look for a 300watt powersupply that is AMD/Intel approved.
Unfortuantly the are a million things that can cause this fault. You'll just have to go through replace/repairing everything one by one until u find the fault.
Good Luck.
-=Stash=-
