I'm running Ubuntu 12.04. I had to reinstall the OS on this computer. I didn't have this problem before. This happens in VLC, and youtube. If I'll play a video in full screen, the screen will go black after about ten minutes, and the sound will kind of repeat over, and over where it went black. The only way to fix it is to restart the computer. I can't even get my screen to go back on after that happens.
Now it's not only when it's full screen. When I watch youtube videos at their regular size my screen will go black after a while, and I have to restart it.