one offhand idea and one from a forum:
1) clear cookies (on firefox, go to tools, clear private data, and be sure to have checkmarked "cookies").
Try ...
2) add "&fmt=18" (without the quotes) at the end of the youtube URL in your browser's addresslist after selecting a movie...
Try ...
2 I took from a forum, apparently, this will try to load a different version from the requested clip, and seems to have resolved a few people's problem with loading..
Does any of this help?