Screen flashing and freezing.

Hi! I recently had to reinstall 7, because the computer kept crashing in the middle of the simplest things. Say, I'd be reading a Wikipedia article, or watching a YouTube video, or playing a game, and the screen would all of the sudden start flashing for a few seconds, then it would just go black (70% of the time.) On occasion, it wouldn't go black, but instead becomes the color of the most...