Freezing iMac's are usually accompanied by graphic artifacts and very consistent screen lockups. The machine actually continues to operate, however the screen no longer is refreshed except for mouse movement.
A great test, is to enable your seconds display on your clock so that you can see if your screen freezes for sure when you are testing or verifying.
Quickly changing screensavers through the selection menu for 5 to 15 seconds in a row is usually guaranteed to freeze the system. Another is to rotate a photo quickly many times in a row within iPhoto.
The solution is not the updates 1.2 or 1.3, as they have 0 effect for the true freezing that is plaguing the iMac aluminum's.
The problem with artifacts and freezing on the iMacs is in fact video memory which requires an entire logic board (motherboard) replacement at your nearest Apple Store (repair center).
I was not too surprised with this as the solution, as I had 100% trouble with fresh installs of Tiger and Leopard as well as freezing when during the initial loading of the Install CD's. This was a strong indicator that the hardware in the machine or firmware was at fault.
Firmware tested and verified aside, the local Apple Store has my iMac and is replacing the logic board and will have it ready in 3 - 5 days.
I would strongly suggest all users do the following:
a) Contact Apple Care and run through their standard testing and verification process. This is required before a local repair store will even consider replacing hardware.
b) Upon authorization and suggestion by Apple Care to have hardware replaced, run it to your local store immediately to get this started as quickly as possible.
Another helpful tip, if you need it resolved FAST, then purchase Apple Pro Care, which is an expedited repair service option, which I was told places you first in line and almost guarantees a 2 day turn around for repair.
Hope this was of help.
Good luck. -- Oh, and they told me at the store, this was the first time they had seen it. 😉
