I had this problem with my iMac either failing to boot up (it sometimes took three goes) or freezing in the middle of an operation. The guy in the Apple store t hought it may be the hard drive
I verified my hard drive via disk utility in the start up and this seemed to cure the problem.
You need to do this from 'start up' not from System utilities when it is booted up. This is the procedure:
Shut down the iMac.
Hold down Control and R when you press the start up button (before the start up sound comes on)
The computer will show a series of options.
Choose DISK UTILITY
Select your Mac hard drive and try REPAIR
If this operation is not available then VERIFY DRIVE.
When this is finished, shut down and restart normally. All should be ok.
I'm having same probs. My 2 year old 27"Imac freezes. It started right after my care plane expired. I took it to apple, said my logic board is fried. I paid $100 to retive my data ( my darn external back up drive ws unplugged for a while ). At the time it was completely dead, will not finish boot up, just a white screen and lines. New logic board $1400, lol. I just got a new one for a little more with a super extended care plan. So I start playing with my old broken IMAC, reloaded os and now works almost fine. Only time it locks up is when playing games, even with lowest game video settings. Everything else runns fine, watching DVDs streaming youtube etc... just playing games (wow). I am noticing this one runs hotter to the touch then my new Imac. Heres my plan from this point, any suggestions pls.
Re-paste the heatsink and test fan- if still fails,
Re-flow GPU, last resort Re-ball GPU
Yes, I have a jack plugged in that feeds to my amp. Often after mail comes in I hear a click a couple of seconds after the "mail in" sound has played. Almost like the audio card / processor finishing it's task. When my iMac was freezing, this was the sound I heard before the big freeeze! Then I got a screaming / garbled nosie sound like my Mac dying - seriously.
However, I pulled out the Apple supplied 2 x 2GB of ram and left my third party 2 x 4GB intstalled and I've had about two crashes in 14 months.
Apple had me ripping all sorts of things apart etc (including removing hard drives I needed for work). I knew it was none of the things they were suggesting. Eventually I made the decision to follow my own instinct and try the RAM.
Apple were very helpful btw but way off target. I still believe they know there are hardware issues with the iMacs. Not even a software patch can fix them!