Possible causes:
-Everything not completely seated on the motherboard.
-Overheating
If the system temp gets over 45 deg. C, then make sure the computer's fans are running, the vents are open, the computer is not located in an enclosed space or near a heat source, and there is no internal dust build up.
If those are ok, then remove the computer's cover, and run an external fan. If this fixes things, then install additional case fans. Recommended locations are lower front and upper rear of the case.
If the CPU gets over 60 deg. C, then make sure the thermal compound has not oozed out from between the CPU die and the heat sink. If that's ok, then replace the compound with Arctic Silver.
If no luck, then install a higher capacity heat sink fan.
If still no luck, then replace the heatsink with a more efficient unit.
-Overtaxed or bad power supply
Either test it, or swap it out with a different(preferably higher wattage) unit.
Also, always use a reliable brand, like Antec or Enermax. Off name cheap brands(even high powered units) can have one voltage leg that's insufficient to handle the load.
-Bad memory
Run Memtest86 and if problems are reported then -> remove and reinsert the memory a few times, if using multiple modules then install just one module at a time and try it in different slots, do not use any optimal settings for the memory in BIOS, and swap it out with known good memory.
Also, do not mix parity and non-parity memory.
-Bad video card.
Make sure it's fully seated, it's fan is running, its heatsink is securely mounted, and there's no dust built up on the fan and the heatsink.
If those are ok, then swap it out with a known good card.
-Bad MB.
Check for bulging, leaking, or ruptured capacitors.
If they look ok, then swap it out with a known good MB.
-IRQ conflict with a network card.
If no problems show up for the NIC in Device Manager, then remove it from DM, shut down, remove the card from the MB, and restart.
-Viruses and spyware.
Run Ad-aware, Spybot, and Hijack This for removing spyware.
-Kazaa.
