My 24″ iMac (early 2008, Core2 Duo 2.8GHz) came with MacOS 10.5 Leopard and worked great with it. When 10.6 Snow Leopard came out, I upgraded to it, and thereafter the iMac started crashing 1-3 times a week. The screen would turn black, or gray, or striped. It wasn’t totally dead, which would suggest a power supply failure. It wasn’t a kernel panic (with the multi-lingual “You need to restart your computer” message), which would suggest bad RAM. But it was hung and couldn’t be reached over the network. To recover, I had to hold down the power switch to force a shutdown, then turn it back on.
It happened sometimes while the machine was idling, other times while I was in the middle of doing something. This persisted through the 10.6.4 update, at which point I decided MacOS updates weren’t going to fix it.
I tried a clean install of Snow Leopard, but that didn’t fix it. I replaced the RAM with a different brand, but that didn’t fix it. I had the main logic board replaced under AppleCare, but that didn’t fix it. After some searching of the web, I found the solution. It confirmed my hunch that the video chip was overheating. The video chip (GPU) in mine is an ATI Radeon HD 2600 Pro. Apparently Snow Leopard drives the GPU harder than Leopard does, but runs the iMac’s fans at the same barely-audible speeds. The problem is more likely to occur on iMacs that are left on all the time, as mine is.
The solution is to install a third-party fan control program to force the fans to run faster and keep the inside of the Mac cooler. I installed iMac Fan Control. The iMac’s fans are now audible, but not too objectionable, and it hasn’t crashed in the two months or so since I installed iMac Fan Control. The temperatures are significantly lower than before. Here is a picture of my iMac Fan Control System Preferences panel.
Update: A year after installing iMac Fan Control, the iMac hasn’t crashed even once from the video chip problem. It’s currently running MacOS 10.6.7 plus the Snow Leopard Font Update.