Apple has released an update for iMacs with the Radeon HD 4850 Graphics processor, which addresses intermittent freezing on the systems that people have been experiencing. This update also fixes problems with wake-from-sleep when using Bootcamp on these machines.
We recently reported on this problem, and while some people were able to overcome it with PRAM and SMC resets, the problem persisted for others despite these efforts
This update should address this issue, and is recommended for all iMac users who have the Radeon HD 4850 graphics chipset.
If your computer is eligible for this update, the update should appear in Software Update. It is also available via direct download from Apple's site: Download Link
Have problems with this update? Let us know! http://www.macfixit.com/contactResources