Problem with video juddering on new pc

i recently bought a Dell XPS 8300 (with Windows 7 64 bit) and am experiencing an annoying issue while watching videos. Videos seem to judder/lag and it is especially noticeable in panning/fast motion scenes. Videos are still watchable but it is extremely annoying. The problem also seems to persist on all video filetypes mpeg, xvid, avi etc.... I've tryed various solutions such as codec packs (K-Lite and Sharks Codec), various players (windows media player, VLC, winamp & MPC-HC) and most recently replacing the ati raedon hd 6450 with a Nvidia 9600 GT. None of these having seemed to alleviate the problem and i honestly can't think of what to try next. I running of a off a 20' inch ACER X203H set to 1600 by 900 60hz. Any help on this matter would be greatly appreciated.

Edit- I have also run all Dell's self test and they cameback good. Also the audio is largely unaffected by the judders and stays in sync.

Edit II - I've just tried do a clean windows 7 install and this still hasn't fixed the issue...

My XPS 8300
http://www.microcenter.com/single_ [...] id=0373551