Quality control problems once again, this time concerning Apple's latest iMacs, which seem to suffer from a graphics card problem that causes the computer screens to lock up.

Numerous discussion threads on Apple's support forums report that while the iMac display becomes unresponsive, the Mac itself appears to work normally, indicating the fault relates to the iMac's ATI Radeon HD card.

iMac users suffering the screen freeze can get their Mac working again with a hard restart.

Some users claim the fault appeared after they had installed recent software drivers (v1.1) for the Radeon HD card - others claim that restoring the iMac to factory settings fixes the machine.

While Apple is alleged to be aware of the issue, no fix for the flaw has yet been made available.