HELP?!? - iMac 2011 i7 Screen Flicker issue (OSX 10.8.3)

I have a Mid 2011 27" iMac with i7

Since the latest OSX update (OS X 10.8.3 (12D78)) I am getting a very concerning issue where my secondary monitors and my iMac screen 'flicker' and show horizontal bars (green in colour) the audio then seems to stutter and crash before eventually the computer freezes and I have to hold the power button to shut down.

This happens once or twice a day, however a related issue that happens every few minutes is that just my secondary monitors flicker white in places and go completely black for a few seconds at times too.

I have run the Apple Hardware Test and it reported no issues, I also reinstalled OSX from recovery and it is still happening.

I am hoping (praying) this is an issue with OSX and not my hardware. Can anyone shed any light on this - I'm seeing reports of 'similar' sorts of issues with the latest OSX update. But nothing like the issue I mentioned first (the one that forces me to shut down)

I have a Time Machine backup, is it possible to use Time Machine to revert my OSX to 10.8.2 to see if it is indeed the OS that has the issue?

Thanks for any help you can give!!!

Jay