Youtube videos playing 5-10fps when in fullscreen mode.

[cupique]

I've been having problems with youtube's flash-player after installing mountain lion. Videos play fine otherwise, but if I switch to fullscreen framerate drops dramatically. CPU usage stays low the entire time but video stutters like no other. I did even a fresh install of OS X, but even that didn't change a thing. Any other videoplayer works fine in fullscreen so it isn't about videocard being hosed. It isn't a buffer problem either, because if I let the video buffer fill up, the problem persists. Other flash-based players also work fine. Problem occurs in all major browsers.

Is anyone else experiencing this or is it just me?

Configuration:

iMac 27" late 2009

2,8Gz Intel Core i7
16GB DDR3
ATI Radeon HD 4850 512 MB
OS X 10.8.2