I can only guess that there may be a minor gap between the cooler and the surrounding memory chips. If you go too thick on the thermal pad for the GPU it will create a gap between the cooler and the surrounding memory modules. I had to experiment with several sizes and compounds to find the best fit. It does sound like a memory resistance due to the timing, as it would take a few more cycles for the GPU to activate the fans.

Please also add to guide: DO NOT USE ARTIC SILVER ON THE GPU MEMORY!, yes i know i could just edit it but i'm confident you will read this info^

Thanks Walter!

Prior to improving the thermal transfer on the GPU memory I have seen elevated temperatures and graphic artifacts when pushing the graphics to the max, after doing it this way I have experienced no issues and lower temps. As a side note with all thermal tapes they have a tendency to have a shorter effective lifespan than quality compound, but I don't keep computers long enough to be concerned about it. I would suggest a 2yr replacement interval if you choose to keep it for extended timeframes.

Enjoy!

YES, I F*&^* hate thermal tape when compared to quality compound. With this only exception, the tape is more effective than the stock compound, and it is quarantined to make 100% contact, while dissipating heat properly. I have seen a drop of 15% under heavy use on the GPU since using the tape. YOU do not want to use cheap thermal tape! Standard tape only has a 3-5 watt thermal conductivity, which is equal to or less than a standard compound. You can go through frozencpu.com and purchase Fujipoly Extreme Thermal Pads which have a rating of 11. For use on the 27" iMacs, I went with 100 x 15 x 1.5 and (2) are needed to do completely cover both sides of the memory banks on the logic board.

Please, if you remove the cooler to replace the CPU you have to be aware of the GPU and the gap involved on the memory chips, and the potential of further gaps upon re-installation.

Furthermore, with the GTX775M and GTX780M equipped iMacs, the temperatures are already above normal without removing the cooler. If you try and push these chips with the added memory in the latest generation games you will get artifacts from overheating. Due to the gap between the memory and the cooler you are also unable to use Arctic Silver as a remedy, as the gap is too much, with the logic board being vertical. (Risking seepage down the board, which I have already seen when performing the proper fix on another iMac) The coverage and type of thermal compound Apple is using is not sufficient. In the factory state the thermal compound is centered on the memory, however only 40% of the actual module comes in contact of the memory and the cooler. What I have done on my iMac and a few others so far is use thermal tape.

The thermal compound/paste that is used by Apple on the GPU and memory is less than stellar. I would highly recommend stressing the GPU Memory gap in this guide. As you can see in the images of the cooler, specifically the GPU side, apple uses a white paste compound for thermal transfer. The problem is that the cooler does not contact the memory for the GPU. Once you remove the cooler it breaks the compound in several spots, and it will never be as good as it was unless care is made, and you are aware of this prior to re-installation.