It seems like i have a similar problem with you. The audio keep going but the video stuck and it only in fullscreen HD youtube videos and it has been going on for a while. Ironically, my system spec:-
Athlon II x4 620 @ 2.6Ghz
OCZ 2GB ram 1066Mhz
ATI RADEON HD5750 1GB
i suspect it must be something with graphic driver.