Hi Lorride,
I did try but, after realising that Firefox 5 already is, I think, HTML 5 ready and "joining" the HTML 5 trial I couldn't manage to play anything in HTML 5 so far as I could tell. If HTML 5 is an option set by going into "Settings" in the resolution change menu (bottom right of the video) after one has opted in then I'm stuffed. If YouTube think they can blackmail me into sharing my personal data just so I can make some extra video settings options then they've got another think coming.

Or maybe I just couldn't find a movie with the right options that didn't include an advert.
But thanks for the suggestion. I also tried clearing out my old YouTube cookies just in case there was something in there that was causing trouble but I don't think it helped. I've Googled this and what I'm seeing isn't uncommon. And, as I mentioned above, I can always navigate direct to the HD version of a video and avoid the stutters, albeit at the cost of a few extra keypresses.
Bob.