We get a lot of iMacs in with video card issues. You can tell that your iMac has overheated and affected the GPU (graphics card) when you see stripes, weird colors or other artifacts on the screen. You have two options at this point. The first and cheaper of the two, involves reflowing the existing graphics card and trying to undo some of the damage caused by the extreme heat it has had to endure. The success of this repair depends on how long the graphics card heating issue has been ignored. If the damage isn't permanent you could get a year or more out of this repair. The second method involves replacing the graphics card entirely. This method is costly because these graphics cards are unique to the iMac and are very expensive. Either method you choose, you must still be aware that ignoring heating issues in an iMac can easily lead to LCD damage and hard drive failure.