iMac GPU Freeze

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I’ve been slowly trying to figure this one out. Every couple of months, the iMac (late 2009), so it is now old. Freezes with strange artifacts on the screen. If I were on a Windows machine, I would load the latest video drivers, but that isn’t an option on the Mac. I’ve been going through the console trying to figure out what program is causing the problem. Again if I were on Windows, it would be some kext or device driver problem.
Thanks to the sleuthing, I’ve gotten rid of lots of strange programs that load at start. Like bits of Buffalo NAS Navigator, but now I just got a fault that looks like it might help. Got a ** GPU Debug Inf Start ** and then a big hex dump in the console, so a search of Google gets me to a long thread on folks with these kinds of problems.

Firstly, it seems you need to install smcFanControl to keep you system properly cooled – already had that for 10 months.
Next, prevent launchd from loading any user specific programs – did that, no improvement.
E.g. rename
mv /Library/LaunchDaemons  /Library/X.LaunchDaemons
mv /Library/LaunchAgents  /Library/X.LaunchAgents
and reboot.

 
Next, disable Quartz Extreme, maybe keep things a little cooler – did that, no improvement. e.g.
https://discussions.apple.com/message/9304483#9304483
defaults write /Library/Preferences/com.apple.windowserver GLCompositor -dict tileHeight -int 0 tileWidth -int 0
to re-enable
defaults delete /Library/Preferences/com.apple.windowserver GLCompositor

 
Next, maybe its the new version of the ATI graphics kexts in /System/Library/Extensions ? 
So, downloaded Kext Wizard and installed the 10.6.2 versions of the relevant ATI kexts, ATI4800Controller.kext, ATIFramebuffer.kext, and ATISupport.kext – did that, no improvement.
https://discussions.apple.com/thread/2384136?start=405
OSX Lion + 2011 iMac 27” + Video =…: Apple Support Communities

So off to try some stuff. First theory is that it is because the system is overheating, makes some sense to me, Eidac.de makes a fan control utility that you can try. The main question is what is the right temperature. This iMac at idle runs a CPU temp of 46C which is actually pretty high given builds I’ve done (normally I try for 40C)

My default position is with the slider all the way to the left. Higher RPM is with the slider about halfway, and Super High is with the slider all the way to the right. Mac Running Hot? We Take a Look at SMC Fan Control: Is it a Good Solution to Keeping Your Mac Cool? | NewtotheMac.info

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