Apple released an iMac firmware update this past weekend ... this fixes a number of freezing issues associated with these iMacs. (Aluminum iMacs)
You should be able to run software update and find out if you can use this update. Running the update is simple and explained within the download.
You can also download it manually here:
From Apple's website:
This update fixes intermittent system freeze issues for iMac computers with ATI Radeon HD 4850 graphics and fixes wake-from-sleep issues in Boot Camp.
After the firmware is successfully applied to your Mac, your Boot ROM Version will be:
Operating System Requirements:
• Mac OS X 10.5 Intel
• Mac OS X 10.5.6 or higher
• iMac with ATI Radeon HD 4850 graphics