Apple has issued another software patch for the much-reported software glitch that could cause some iMacs to freeze during normal use.

The problem - and the patch - isn't for every iMac, the company warns. The iMac Graphics Firmware Update 1.0 software specifically targets 20-inch and 24-inch aluminium iMacs with an ATI Radeon HD 2600 Pro graphics running Mac OS X 10.4.10 with iMac software update 1.2.1 or Leopard with iMac software update 1.3 installed.

The patch updates the graphics firmware on the ATI Radeon HD 2600 Pro to improve system stability, hopefully ending the oft-reported iMac screen freeze bug.

"Not every 20-inch and 24-inch aluminium iMac computer requires this update. If you are not automatically presented with this update, your computer does not need it," Apple warns users on its website, Attempts to install the software on an unaffected computer will fail.

Apple released an update that was supposed to fix the problem earlier this month, but it reportedly failed to work for all iMacs.