Many many thanks for your research into this subject. I've just replaced the heatsink in my iMac and the GPU is now running at the right temperature. My heatsink had a split in it causing the GPU temperature to go up and up. After a few minutes, the screen went black. Simply swapping the heatsink for a replacement fixes the problem. No new GPU required.
The articles I read are: https://www.youtube.com/watch?v=qXso8KFG... and Graphic card overheating, CPU fan in full speed, then black screen Haven't tried replacing it yet, but will confirm myself once the part arrives. Anyone else had joy with a new heatsink?
There are confirmed reports that the heatpipe part of the heatsink is failing. Small pinholes have appeared in the heatpipe. This ruins the heat transfer capability of the heatsink and makes the GPU overheat. By simply replacing the heatsink the problem is fixed.
Well I've just had the same problem with a MBA late 2008. The keyboard and trackpad wouldn't work at all - not even at boot time. The USB port wasn't working either - it wouldn't boot from an external DVD drive. I checked all the cables, looked for damage/spillage too - nothing. Then I noticed that the late 2008 model has an nVidia 9400 chip in it. Could the USB ports/keyboard/trackpad be driven from the nVidia chip? I checked out the hardware description on the web and found that it does indeed drive the graphics as well as the USB, SATA, etc. Knowing the nVidia chips around this era had issues with dry joints, I thought a reflow may be my last chance. So, out with the reflow station, let it cool and put it back together. Powered on the MBA and the external DVD drive fires up, the keyboard and trackpad work too. So, if you have the same problems, a reflow of the nVidia chip may fix it once you've tried the cables, etc first. I believe there are many services on the web that offer reflows on these chips....