It is almost certainly the sign of an overheating/failing graphics card. Open the case, try cleaning the fans, especially the one on the GPU itself, but more than likely, you are looking at replacing/ or upgrading your graphics card.
Find out what you have, if you don't know you can open up the windows button, and down at the bottom search for dxdiag, you'll find a program and you can run that, it'll tell you your graphics card. Check around on the net, I like NewEgg and Tiger's Direct, myself. They tend to have the best deals, you can usually replace the card fairly reasonably priced and it's an easy swap and install to do yourself.
Before that, though, be sure to go to the graphics card website and download the latest drivers, it could be something as simple as an outdated driver, but my gut feeling is that the card is overheating/starting to fail and you'll want to run through those initial tests and line up a replacement while it is still functioning enough for you to do it.
Good luck with your issue.