Apple has released new firmware for 27-inch iMac systems, this update promises to fix the graphics issues many users were suffering from. You can download the update at this page.
Updates the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to address issues that may cause image corruption or display flickering.
The iMac Graphics Firmware Update will update the graphics firmware on your iMac.
Do not disturb or shut off the power on your iMac during this update. Loss of power could result in your iMac failing to start up.