On Ubuntu 8.10 in Firefox with Flash Player 10 YouTube videos were stopping after 2 seconds. I tried a few things like clearing the cache and killing pluseaudio but for some reason disabling the Flash plugin then restarting Firefox and renabling Flash got things started again.
http://ubuntuforums.org/showthread.php?t=598034&page=2
http://ubuntuforums.org/showthread.php?t=598034&page=2
