Sorry, been out of action for a few days. Google didn't turn up any specific issues with your video card for me.
The single case of sluggishness sounds a lot like your RAM was maxed out. Not what you'd be expecting in 6GB under normal use.
I'm not using 11.10 but I know a few people who are & haven't heard any screams of outrage with Youtube & FF yet. I'd try another browser & see if you get the same thing happening.