That is most often a bad video card or power supply. If anything is overclocked set it to stock speeds, make sure the case and heatsinks on the video card and CPU are clean, test the card in another system, test the power supply in another system, test new card and/or power supply in yours. Before buying replacements, you should also try removing the video driver with driver sweeper and installing new drivers, if that does not help, try it with a clean Windows setup. If that does not help, you will need to swap out hardware to find the exact cause.