Hi, I can't advise anything to do with your Mac computer, but I do know that after this latest Flash Player update, my perfectly fine video on YouTube took a big turn for the worse. Just the same as what you are seeing, a lot of buffering (stopping) going on, where as before, video was zippy. There are minimal settings for the Flash Player from Adobe that Windows computers use, and I am not up on what a Mac would use as far as a plugin type thing so hang on and hopefully someone will be able to help.
Have you searched the Mac forums for help with YouTube?
Note> Technically you would be best off in our Apple forum so I will probably move your thread there.
We allow just 1 thread per issue on the same computer, so please do not start any new threads for this problem. Thanks!