Apple is looking into a flood of complaints about software bugs that cause its newest line of iMac desktops to freeze up.

Introduced two months ago, iMacs with the aluminium casing have been freezing under a variety of conditions, but common characteristics include inactive mouse and keyboard; continued background processing, such as music that keeps playing; and the need for a reboot. The screen remains lit, however.

"Whatever was going on at the time continues, so if you were listening to music it continues to play," reported user “tekn0ph1sh” on Apple's support website. "The mouse pointer moves, but you cannot select anything. Tabbing through apps doesn't work. Volume keys and brightness are also unresponsive. The cap locks button turns on and off."

Although the volume of messages citing locking iMacs swelled this week, tekn0ph1sh's thread dated back to late August, about three weeks after CEO Steve Jobs launched the new models during an event at Apple's headquarters.

Theories abounded as to the cause, but many users are convinced that it would be traced to the iMacs' ATI Technologies graphics cards and drivers. Apple shifted to ATI hardware for its iMacs, and dropped its former supplier, nVidia. "I don't know why for certain, but I suspect that there is some interaction between some games, the [ATI] graphics card and the drivers for the card," said Al Van Malsen on the same support site forum thread.

AMD, which owns ATI, did not respond to a request for comment.

An Apple spokesman acknowledged the problem, and said it may have been caused by a September software update. "A small number of iMac users have made us aware that a recent software update issued by Apple can sometimes cause their iMacs to freeze, requiring them to restart their iMac," the spokesman said. "We are tracking down the root cause of this bug, and will issue a software update which corrects it as soon as we can - most likely later this month."

The 1.1 update, released 13 September, came with only a terse explanation that it provided "important bug fixes." Numerous users on Apple's support forum mentioned 1.1, and some even claimed that they had solved their problems by rolling back to the original configuration. Others, however, disputed those accounts.