Apple issued an iMac EFI Firmware Update 1.4 which specifically addresses an intermittent system freeze that was occuring in iMacs with the ATI Radeon HD 4850 graphics card. Once you download the software update, you must launch it manually it will not be installed automatically. To complete the firmware update process, please follow the instructions in the updater application (/Applications/Utilities/iMac EFI Firmware Update.app). The updater will launch automatically when the Installer closes. The update also fixes wake-from-sleep issues in Boot Camp.
