sturm could be right it may be a power supply [P/S] problem, there could also be a host of other reasons it could be happening, his suggestion to swap out a P/S with a known good one is a good suggestion, because you can discover if the problem is solved by doing so.
But if you don't have that option and end up buying a replacement P/S, you also need to be taking into consideration, are we talking a name brand computer such as Dell, Gateway, HP, Compaq, Etc. which may take a specific replacement size and type of P/S, so what type computer do you actually have?
If the computer is over 3 years old, its a good possibility it very well could be a power supply problem, your local computer repair store will have a power supply tester to see if there is an output problem with the P/S and if it is, they can reccommend the replacement you need.
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