-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 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.
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.