I have been experiencing this problem for some time as well. I have a 20" 2007 iMac 2.0 C2D, 4GB Corsair RAM, and had the OEM 250GB Seagate drive. I bought the computer used, so I don't know it's history. When I first got it, I installed Snow Leopard on it, and when Lion was released, I installed that. Only occasionally would I experience a hang - once every couple of weeks. Generally it was as other people have described - a beach ball and non responsive UI with the exception of the cursor. Sometimes there would be some UI artifacts left over. I tried several of the things other users have - used the restore disks to try 10.4.10, and never experienced a hang. Then used the 10.5.1 drop-in-disc that came with it new (upgrade DVD) and never experienced a hang. As I upgraded to Snow Leopard and ran Software Update I upgraded to 10.6.7 and still never experienced a hang frequent enough to speak of.
I became used to the functionality of Lion and enjoyed the full screen functionality of VMware Fusion, so I would always end up upgrading to 10.7.
As time went on, the hangs and freezes became increasingly more frequent. So much so, that the computer was almost unusable. I would start the computer and visit a Facebook page that listed several friends, and it would hang. It would also do this with 10.8. It got to where it would hang 4-5 times per hour and require a reset. I tried turning the fans up to see if it was a thermal issue a few months ago, but that made no difference, as some have noted here. I also ran memtestosx in single user mode and it found no errors.
Today, I disassembled the computer and installed a 120GB OCZ Vertex II SSD as a last ditch effort to see if I could salvage the poor iMac. While I was in there, I blew out the dust that had accumulated (which wasn't a great amount) and removed and reinstalled the RAM. I assembled the computer and installed a clean copy of 10.7.3 from the App Store. So far, the computer has not hung one time. I know this does not match the above theory, but it makes no sense if it is purely a video driver issue that the frequency of the hangs would increase over time. There must be another factor involved. So far, the computer has worked better than it has since I have owned it. I would experience a hang within 15 minutes of using it before. Now I have used it all day, and it has been as smooth as butter.
I just wanted to throw this out there - perhaps it will help someone else.
Here is the hardware and video card info:
Code:
Model Name: iMac
Model Identifier: iMac7,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache: 4 MB
Memory: 4 GB
Bus Speed: 800 MHz
Boot ROM Version: IM71.007A.B03
SMC Version (system): 1.20f4
Serial Number (system): XXXXXXXXXXX
Hardware UUID: XXXXXXXXXXXXXXXXXXXXXX
ATI Radeon HD 2400 XT:
Chipset Model: ATI,RadeonHD2400
Type: GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 128 MB
Vendor: ATI (0x1002)
Device ID: 0x94c8
Revision ID: 0x0000
ROM Revision: 113-B2250A-207
EFI Driver Version: 01.00.207
Displays:
iMac:
Resolution: 1680 x 1050
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Built-In: Yes
System Software Overview:
System Version: Mac OS X 10.7.3 (11D50)
Kernel Version: Darwin 11.3.0
Boot Volume: Macintosh SSD
Boot Mode: Normal
Computer Name: XXXXXXXXX
User Name: XXXXXXXXX
Secure Virtual Memory: Enabled
64-bit Kernel and Extensions: Yes
Time since boot: 2:04
