Apple has released a firmware update for the new 27-inch iMacs which is supposed to address the kind of screen flickering and image corruption users have been complaining about. It's not clear if the update solves everybody's problem, though.The new 21.5- and 27-inch iMacs came out in October, and very soon thereafter some buyers started complaining that their screens would flicker or flash at more or less random intervals. By last week, there were 85 pages on the Apple support forums dealing with "New iMac 27inch screen flickering/tearing/shutoff."
Yesterday Apple released the 27-inch iMac Graphics Firmware Update 1.0, which "updates the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to address issues that may cause image corruption or display flickering." But apparently the update isn't a sure fix: the information page on the update says that "if your screen remains black after applying the updater or if you continue to experience image corruption or display flickering after successfully completing this update, contact AppleCare or an Apple Authorized Service Provider."
In that forum topic, which has now grown to 105 pages, several people are reporting that the update didn't help, or only helped temporarily. And the imac.squeaked.com site, set up to document the flickering screens and other display oddities, has added a "Flickering after Firmware Update" page with 15 reports so far.
Some, however, were able to get it working after resetting their machine's PRAM (parameter RAM), a time-honored fix for Mac glitches.
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So putting the available information together, this seems like the best way to proceed:
1. Quit all open applications. Apple's instructions say to do this before even downloading the update, much less applying it.
2. Install the update. (You'll need an admin password.) That line I quoted above about "if your screen remains black" would seem to imply that the process makes your screen go black for a short time, so don't panic if that happens.
3. Shut down your iMac and unplug everything, including the power cable. Wait 30 seconds.
4. Plug the power cable and the keyboard back in and restart your machine, holding down Option-Command-p-r until you hear the startup chime three times. Then let go of the keys.
5. Once the machine has started up, plug your peripherals back in.
If none of this helps, contact Apple as suggested. Several users are reporting getting brand-new iMacs at the end of the process -- no word yet on any problems with those.