I've the same problem on fullscreen and with my Q6600/8800 GT.
nVidia drivers 185.18.04.
Jaunty 9.04 x64.
The problem is "showed" with HQ videos
(LQ videos are too little for stress flash plugin).
The worst problem is the video tearing/vsync-off, all videos affected!
Please nVidia, fix it if you can...