I had similar problems when youtube decided to start using HTML5 instead of flash. To see if your youtube is doing the same, play a video and then right click on it, if the last option is "about flash player" I cannot help, someone else plz, but if it says "about HTML5" that might be the case. HTML5 is very unstable on some people. So to go back to flash you have to do stuff, google "how to watch youtube with flash player on [your browser]" or something similar.
Hope I helped. If I did please mark this as the solution, it helps people (and me