Yep OP, been having the same issues. Videos that have played for a couple of years with never an issue are suddenly cranky if they will play at all. And this happens on multiple machines so isn't a localized issue. My guess is it is either an issue with the newest version of Adobe Flash Player or YouTube themselves have broken something. Either way my solution has been to find as many of the videos I like to watch on an alternative service such as Vimeo.