Need help fixing video "stutter"

[RetroGamer]

ok all, I'm running Windows 7 x64 Pro and have a new issue that I have no idea how to fix. (When running same hardware on Vista - both 32 and 64 bit, everything worked...)

My video - be it from a DVD or Blu or from a youtube video / mpg from my hard drive - stutters. Meaning, the sound plays back fine, but the video, every second or 2, 'stutters' then snaps back, only to stutter again. Happens when playing video from itunes (a digital copy).

I have updated to the x64 NVidia driver (latest) as I'm running a GTX 285

I have installed DirectX 9.0c

I have installed the Shark007 codec pack

The problem occurs with Any player (VLC, TMT3, iTunes, Media Player and internet explorer - all tested with same result).

Output is via HDMI - screen resolution is 1920x1200. Refresh rates match.

Any help greatly appreciated...