I had multiple problems with the iMac (late 2006 Intel Core 2 Duo 24")after a few years of use. The computer was running extremely slow even after all manner of fixes including bigger HD, resets of everything, Drive Genius fixes, etc. Then it started freezing in mid opening of applications with a distorted image of a half opened application plus lines through the screen. It became unusable with no fixes from Apple Store recommended other than possible logic board.
I opened it up again and thoroughly cleaned with compressed duster - fan blades, ducts, conductor and cooling fins. In addition the GPU was blown out as well as possible. Finally the gap in the duct between the CPU cooling fan outlet and the copper heat conductor was sealed with aluminum tape and a bit of tape was used around the openings in GPU to keep air flowing through it. (Note the iMac 24 MA 456LL/A that I have is quite easy to open. You do not have to use the credit car to pry back the levers found on some of the other iMacs.)
The unit was reassembled and smcFanControl was downloaded and installed. Before installation the GPU was running over 105˚F with no heavy use. After smcFancontrol even with heavy use the GPU rarely went over 98˚. The problems have disappeared and all seems well. The cost of repairs was $4.50 for a roll of tape of which I used about 6".