iMac freezing after Mountain Lion Upgrade

Sorry, i was more focused on wondering if you think the video card replacement was the fix...

I had the same problem on a 20" 2008 8,1 iMac.

3 reinstalls of Mtn Lion, including hard drive wipe and clean install, PRAM resets galore, same for SMC, new RAM installed and almost resigned to either a new video card, hard drive or computer. Three days downtime etc. Noticed it was heating up just a tiny bit more than when running Snow Leopard. Free install from: www.derman.com/iMac-Fan-Control,

open from System Preferences.

It was a scarcely noticeable difference in running temp to me, but that was the issue. Day 4 and no issues since. It might not be the problem but its free and it might be the cure.

Similar (or same) issue happening to me. 27" iMac 3.06 Ghz Intel Core 2 Duo is freezing up on average once a day. When it freezes, both my mouse and trackpad can move the pointer around the screen but I can't click on anything and I can't do any multi-finger gestures. All I can do is hold the power button down till it shuts down and then restart it. It's starting to get more and more agrivating. I can't remember if it started doing this a few times before I installed Mountain Lion, or if all of this started just after upgrading to Mountain Lion. I sure would love to know how to fix this issue.

Yep. I took it to the apple store and it was my video card as well. booo..

Read my prior posts to this question - different problems can have similar symptoms. However, mine had this very problem and it went away simply by changing to an Apple screen saver. I would also turn off non-stock programs that may install at boot-up and/or may be running in the background. Check Activity Monitor to see. I still think a lot of these problems are programs that don't awake from sleep properly (perhaps crashing/confusing the video drivers). Newer video devices may handle the situation differently than older ones (and hence are less susceptible), but that doesn't mean the older video couldn't work if configured properly. As I've said, mine now works... It costs you nothing to try this first before you take it back.

Hello, I may be too late or this may not work but I was told my Logic Board had failed after my 2008 iMac started crashing and having screen glitches. This was just after a Mountain Lion upgrade. I refused to believe this and after much farting about I changed the screen resolution. I've not had any problems since.

Hello, all. I had similar issues: mid-2011 27" i7 iMac, was running Snow Leopard without issue. Just took it to the Apple Store for a hard-disk replacement (part of the recent Seagate recall), and the machine ran fine afterwards.

Decided to upgrade to Mountain Lion to get the benefits of iCloud. Almost immediately after upgrade, my machine started to hang catastrophically (sometimes indefinitely). Repaired permissions, rebooted, reset SMC, but no change.

After a few searches (including this thread), I did a quick search in Activity Monitor and discovered that AddressBookSourceSync was eating nearly 100% of CPU resources. A quick search led to this:

https://discussions.apple.com/thread/3355782?start=15&tstart=0

Followed the instructions from Isday (about halfway down page), and this resolved the issue entirely (so far).

Seems that iCloud accounts might need to be disabled and then re-enabled (and re-sync'd) to get everything running smoothly again. Had a similar issue with iPhone 4S after upgrade to iOS6; iPhone would not publish Calendar changes to iCloud (iPhone could see changes made on desktop but wouldn't work in the other direction). Disabling and then re-enabling Calendar in Settings fixed the problem (again, so far...).

Hope this helps someone...