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: