Does it have an old power supply? They wear out after a while of course, and before they die, sometimes there can be behavior such as that. Could be a virus maybe I suppose. Have you added anything lately, such as RAM? Improperly matched banks of RAM could cause it to have fits, but that would result in something more obvious, I think you would notice that sooner than later in that particular case. What changes have you done lately, if any, that you can think of, hardware- or software-wise. Could be a conflict of drivers.