Is it just in browser? Flash or any video? Tried Youtebe in HTML5/WebM? Wonky hardware acceleration settings? Faulty GPU hardware? Insufficient power from the PSU? Could be any of those, can you test a video in a stand alone player like VLC or Mplayer? Can you test with full acceleration on and off? Just trying to narrow things down a bit. https://www.youtube.com/html5 you can find them easier using http://mycroft.mozdev.org/search-engines.html?name=Youtube+WebM