If the power supply is old (or even sometime relatively new) this can happen.
Is it happening when you're in Windows? If it is, it might be spyware or a
virus and it would be appropriate to do a system scan. Usually though, this
kind of problem would be due to a faulty/damaged power supply. A quick way
to test the power supply is to remove it from the case and disconnect any of
your components from it. Now, on the power supply, you should see a big
connector that used to connect to the motherboard. Take a paperclip and
stick it into two holes so that there is a connection between the green wire
and any black wire. Now plug the power cable back into your power supply and
turn it on. Now you get to sit and watch/listen to the fan for a while. If
it stutters, restarts, or stops, it's definitely the power supply and you
should buy a new one. If not, what are your system specs and what wattage is
the power supply. It may be that it simply cannot provide the power that is
needed by the system. Best of luck.
