I was having the same issue for a while with my old iMac (17" G5, no iSight). Mine was part of a batch that had an extended warranty to cover getting the logic board replaced, but even after getting that fixed, it still tended to overheat and shut down. I managed to fix it myself, but the solution isn't pretty, and is not for the faint of heart. First off, I would suggest opening it up and blasting out all the dust that has been building up in there over the years, especially if it has never been cleaned before. Cans of compressed air work wonders, especially on tightly packed systems like iMacs and laptops. Make sure you blast out the CPU heat sink, easy enough from the bottom of the system. If it keeps overheating after cleaning it, and the caps seem to be in good shape, you're welcome to try my solution if you aren't afraid to get your hands a little dirty... I went out and picked up two 120mm(iirc) computer fans, cut two holes in the back panel of the iMac case directly on top of the cpu heat sink, and...