In cruising the web, there are videos galore, but intermittently, there is no video (dark screen) although the sound is fine. For the most part, I think this mostly happens from videos in a website as I don't think I've had any trouble seeing most if not all videos directly from youtube for example. Not sure if this is a problem with El Capitan or another problem that doesn't have anything to do with 10.11. I have the latest beta although I've noticed this problem with earlier beta versions of 10.11. Any ideas as to how to correct this problem?