I'm getting this on my iBook from time to time as well, especially on high-res flash players.
I don't really know the source of the problem, but it definitely hasn't got to do with the internet connection's speed here. (24mbits/s)
I do however, notice a large increase in speed when the video's fully loaded, though, so you might wanna wait a few minutes for it to be fully loaded, and you might expect the framerate to go up.