It could possibly be a heat issue. Are you sure that all your fans are functioning and the heatsink is on properly? it could be even a disconnected fan.
another possibility, is that your powersupply could be dying or just not producing enough power to keep the system stable..
i belive your right in thinking it isnt a virus (although i could be wrong)
Hopefully that helps, or someone more apt to deal with this problem with post?
