iMac Video adapter failing?

Hi,
I have a 2007 iMac running OS X 10.6.8.
My screen keeps freezing and the only way to deal with it is to force shut down.
Applications such as Skype and iTunes keep running. Mouse cursor is moving but everything else is frozen.
I reinstalled Snow Leopard, ran the disk utility (repaired permissions,repaired disk),ran Apple Hardware Test,which didn't find anything wrong with the hardware...but the computer keeps freezing.
Any thoughts? Thanks!

Freezing can also be caused by bad memory or a bad hard drive. Since your iMac is 4 years old, I would suspect the hard drive first. The Apple Hardware Test is not very good at finding hard drive problems. Try running Disk Utility from your install recovery DVD and see if it comes up with any errors.

I also thought of the hard drive but I did run Disk Utility from the install DVD and it didn't come up with any errors.

I doubt it's the memory that's causing the freezing. Also, memory glitches usually cause the system to reboot. It could be software but that seems unlikely also. Try booting the system into Safe Mode and let it run awhile that way to see if freezing occurs. Keep making backups just in case.....

Thanks! I'll try that.

Booting in Safe Mode didn't help...
I wonder if there's any way other than Apple Hardware Test to check what's going on...

I have an iMac about the same age doing the same thing. I think the issue is the graphics chipset overheating and freezing the computer. I know mine can be replicated loading World of Warcraft for a few minutes. I could get the same results running a full screen video or iTunes visualizer - anything that will stress the video card. It's possible just increasing airflow through the computer might keep it going a while.

I guess I'll have to take it to Apple...