Ok,
Their was some advice on the Nvidia Forum that gets the Frame Rate back to normal when it drops very low.
I was sceptical that it would work. In fact it DOES work.
I played the game till the Frame Rate dropped to an extreamly Jerky condition. Maybe 10 Frames a Second.
"ALT TAB" back to Windows Dekstop. "ALT TAB" back to the game.....
Guess what ..... It was back to FULL frame rate again !
Now what could possably be causing such a problem were a simple ALT TAB out of the game and ALT TAB back into the Game and the frame rate goes back to normal ?
Very weird.
Charles
Alt-tabbing would revert the environment from a 3D profile to 2D and back to 3D theoretically allowing your video card to reinitialise the 3D settings so it would make sense. Unfortunately that doesn't work for ATI cards though

Btw Charles, since you installed MSI Afterburner, enable the on screen display, that way you'll be able to get dynamic info on your clocks in game.
And for those of you who suggest that it might be a throttling related issue due to heat it CLEARLY isn't, typically video cards don't operate at clock throttling temperatures even on stock fan profiles unless they're running some kind of intense burn in application, Crysis 2 doesn't have any where near the capacity for stress.
Finally, this is just as much a problem for AMD and NVIDIA to solve as it is for Crytek, so keep an eye on the change logs for your video card drivers.