That problem, as you have guessed, could be caused by many hardware and/or software problems. The real troubleshooting starts when(and if) you can establish any patterns: do specific tasks seem more prone to this, are there any other indications of trouble. If this PC has been in use for a while, with no changes or upgrades, then hardware is more likely. From my experience, in order of likelyhood (is that a word?): 1) check the fan in the powersupply or on the cpu. If they stop or are running very slowly (due to dust, age, etc.) your system will overheat and can act very wierd or lock-up. 2) video card failure. 3) RAM failure or incompatible multiple modules. 4) bad AC - I've seen wiring problems that left fluctuating line voltages or no ground connection cause this problem. 5) bad power supply OR too many devices added to an inadequate one. 6) an improperly seated card or connector inside the case. This can happen following upgrades/changes, especially if the system was moved. There are more, of course. Let us know what is found. Other important data is: what video card, size of case, if "overclocked" cpu, recent mods, etc. good luck