By Aidan Malley

Published: 08:40 AM EST

Evidence is mounting that the frequent video lockups experienced by some owners of Apple's all-aluminum iMacs may be the result of a faulty graphics video component or overheating, rather than a software bug.

One owner speaking to AppleInsider noted that the freezes plaguing his new 20-inch iMac have seemingly vanished ever since Apple replaced the Radeon HD video chipset in the system, suggesting that Apple's recent iMac driver updates are not the ultimate solutions to the issue.

"So far that [repair] has fixed it completely," he says, observing that the iMac has been stable for two weeks since its return. "We'll see if the same issue starts back up again."

This and other reports provide increasing support for beliefs that the lockups and related symptoms are caused by a hardware issue, or excessive heat inside the all-in-one chassis. Users often report graphical corruption in the operating system as a precursor to the freezes -- a sign some PC users recognize of a video card pushed beyond its safe operating temperature. In some instances, the visual artifacts become increasingly likely as time goes on.

"My lockups were normal looking at first, but eventually started to spew out graphical artifacts, the same type of [problem] I used to see when overclocking graphics cards," the iMac owner said.

AppleInsider itself can attest to the symptoms, which first manifested in a review unit weeks after the August release. A sure sign that the system is near a freeze are very brief white streaks that dart across the screen -- a behavior witnessed with high-end video cards running past their limits. Visual corruption in certain programs, especially those that require a full-screen mode, also hint at an impending lockup.

The issue further appears to be disconnected from the operating system, as it also manifests in Windows while using Boot Camp (which, incidentally, incorrectly lists the video card as a Mobility Radeon HD 2600) and also appears regardless of the version of Mac OS X installed. iMac users in Apple's support forums complain that upgrading from Tiger to Leopard often exacerbates the problem, freezing when a game, screensaver, or a similarly more intensive task than the standard Leopard desktop is activated.

While the problems have not always been consistent and are sometimes resolved by downgrading to Tiger (which offloads less work to the graphics processor), the freezes are prompting an increasingly hostile reaction from buyers, many of whom are also angry at Apple for failing to either inform customers or technicians about the existence of the flaw, or else to offer a truly permanent fix.

"I've gone from frustrated, to upset, angry, furious to disappointed with my iMac," reports one user from Apple's forums. "It basically sits in my office collecting dust. I don't want to drag it all the way down to the Apple store, because I know they will only exchange it with the another defective unit."