Update your firmware by downloading and applying the iMac ATI Radeon HD Graphics Firmware update.
This update will automatically be offered to affected computers via Software Update. When downloaded from Software Update, the application automatically starts. Follow the onscreen instructions to apply the update.
If you manually downloaded the update using Safari, it will download to the location specified in Preferences/General. If Open "safe" files after downloading is selected, a disk image will appear on your desktop. Open it and click iMacGraphicsFWUpdate.pkg. The updater will be installed in /Applications/Utilities and will automatically start. Follow the onscreen instructions to install the update.
If you attempt to install this update on an ineligible computer, the following alert displays: "Your computer does not need this update."
If your ATI Radeon HD 2600 Pro or ATI Radeon HD 2400 XT cards have already been updated, you'll receive a message indicating that your firmware is already up-to-date.