It is observed that the upper left hand corner of the iMac has some sort of an air pocket that traps extreme heat from the graphics card there. And shortly results into the damaging of the screen/ graphics card.
The simplest way to enduringly fix your overheating 24″ Imac (iMac6.1 iCore 2 Duo at least, and several other models) requires two steps.
First is to download and install smcFanControl 2.2.2. Set it up to provide minimum fan speeds of 2200 rpm for the HD and 2800 rpm for the CPU, leave the Optical Drive fan at 600 rpm to minimize dust accumulation.
Next is to obtain a 12 vDC enclosed squirrel cage fan like the San Ace B76. Hook it up to any common 9vDC/AC adapter (around 10 vDC) to run it cooler.
Tape it to the rear left corner of the back (3″ from the side) so it sucks air out of the hottest point of the back slot and propels it away upwards.
Your iMac will now be fixed since the air slot temps will hardly ever exceed 106ºF (40ºC) again and your graphics card will stop cooking.