Here are 3 possibilities that I can think of:
1. Try reseating your video card - the pins may not be making contact properly (or they could be damaged and thus you'll need to replace it (try other others first before replacing)).
2. Faulty/buggy drivers are another possibility. Try running in safe mode and see what happens. Safe mode uses very basic drivers provided by Windows. If safe mode does work, check the device manager for any hardware-related problems.
3. Your power supply is inadequate (not enough watts or too low of amps on one of the rails). If this is a case, you'll need a new power supply that provides more watts.