In its move to tackle the current issues affecting iMac 27", Apple released a new firmware update, specifically dedicated to display problems:
Updates the display firmware on 27-inch iMac systems to address issues that may cause intermittent display flickering.
The 27-inch iMac Display Update will update the display firmware on your iMac. Do not disturb or shut off the power on your iMac during this update. For detailed information about this update, please visit this website: http://support.apple.com/kb/TS3207.
