In all seriousness, have you ever cleaned out your iMacs "ventilation ports" in that 6 year old machine? Those ports intake and exhaust the hot air that the internal cooling fans move, and over time get plugged with dust. I would take a vacuum cleaner, while the iMac is OFF, and vacuum out the ports on the bottom center and the upper left back of the iMac and see if allowing better cooling thru those ports speeds up your unit.
Hope this helps
You could try a few things, may or may not help, but worth a try.
1) Boot into Safe Mode. Press and hold the Shift key immediately after the Mac restarts and hold it until you see the Apple Logo. See if the same happens in Safe Mode. Then Restart the usual way.
2) Boot from your Snow Leopard DVD and go to Disk Utility. Check or Repair the Disk. Quit Disk Utility and Restart.
Well Mac SA took it away for 2 whole weeks less than 6 months ago and tested EVERYTHING, so I guess they had it in bits then. The verdict was 'nothing wrong' with the hardware, but I can see there's still a problem! And yes, it's been cleaned several times over the years, but thanks for the thought.
I have had trouble with my graphics card on more than one PC before, and wondered if the Mac could be having the same problem?
I'm sorry to say I have to use Disc Utility at least once a week to 'repair' when the whole thing falls over too.
So you think it might be software? Mac SA has also wiped the whole thing clean and reloaded everthing more than once in the last 6 months. The programmes seem fairly stable (compared to PC anyway) but still the problem with viewing windows!