Chosen Solution
Here’s the steps to replace your GPU card: iMac Intel 24" EMC 2111 GPU Assembly Replacement. But, I wouldn’t replace the GPU card yet. Given the age of your system I suspect yours is quite dirty inside and needs a good cleaning and the thermal paste on the CPU & GPU needs to be replaced. I would start there cleaning out the dust and redoing the thermal paste. It just maybe the issue is heat. To help monitor things I would also install this app: Temperature Gauge Pro so you can see the before & after conditions.
Successfully reflowed the 8800GS GPU after removing the Graphics card from the main logic board and heat sink. Cleaned off the old thermal paste with ISO alcohol.Put a little bit of flux around the GPU. Put foil under and around the rest of the boardPut the board in the oven from ambient to 160℃. Then upped the temp every 3 minutes in steps of 20℃ to 240℃Turned off the power to the oven letting the whole logic rest inside oven with the door closed until back to ambient temperature… Cleaned the board of the flux residue with ISO, applied new thermal paste, reassembled the system … Tested and back running again :)
