Apple has released a firmware update for iMac owners, the update fixes several issues to improve the stability of iMac computers.
Apple notes on their website. After the firmware is successfully applied to your Mac, your Boot ROM Version will be: IM71.007A.B03
You can confirm the version of the BootROM installed on your computer using System Profiler. 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.