iMac 27" suffers graphics issues when waking from sleep

Hi,

I wonder if anyone can help. I have a late 2013 iMac 27 that when awaking from sleep the screen goes from black to pixalated mess (you can just about make out the login box)..

It also has a secondary screen connected via a thunderbolt cable to a Dell (VGA connector) screen. The secondary screen DOES NOT get corrupted

Recent s/w changes i have made are: Update to VMWare Fusion & install of Universal Media Server. Vmware is running a single instance of Windows 7. Updates have both been backed out with no recovery of the situation

Tried claring PRAM etc - no change

The problem does not go away - every time the machine enters sleep mode and i awaken it (via mouse or keyboard) the problem persists

A hard reboot (power off and on) clears the problem until the machine next goes to sleep

specific part number / spec is as follows:

IMAC 27/3.2QC/2X4GB/1TB/GTX675MX
Part Number: MD096B/A

If anyone has any ideas - would be much appreciated.

Thanks and regards,

Stuart.