Choppy vidoe playback with 4870

I am having real problems with video playback since upgrading to a hd 4870. Games run like a dream but any cut scene or intro is terrible , the video seems to hang whilst the sound continues then will suddenly restart at the point the sound is ( if that makes any sense ) This also happens while trying to view video files , i have dowloaded the most up to date drivers but with no success , any help would be greatfully recieved , cheers

Can you post the rest of your PC specs ie Mobo make type current bios and so on.