try messing with the refresh rates found in the video options area

sometimes if the refresh rates are out of sync it can do this

i would say it is most likely monitor related.  do you have a lcd or crt monitor?  can the lcd handle multiple resolutions or is there a "best" res.  is the res of the monitor in windows the same as the res selection in game?

does it still get the shakes if you play a long game on lower graphic quality? 

if it starts shaking and you save the game.  does it still happen after being away for a few hours?

if everything is high and it only happens after you've played for a while, maybe the video chip sets are over heating.  i know crysis is a hog, but who knows.  maybe a fur ball took up residence and its heating things up.

Yeah fairly new monitor... 1yr half old.

Does happen more often after it starts... even after re-loading the game only take half the time as before.

Otherwise upon closer inspection my computer had become quite dusty internally - far quicker since my last clean.

Anyway - thanks everyone for the help but I'll get some compressed air and give it all a good clean and that should limit the problems.

All the best