Screen Blacks Out After Fans Run at Maximum
https://www.youtube.com/watch?v=qXso8KFG78M

see post from tommyjames 337 half way down the page

https://www.ifixit.com/Answers/View/126496/Graphic+card+overheating,+CPU+fan+in+full+speed,+then+black+screen

"Okay you guys! After months and months of trying, I have found the true solution to the overheating graphics card. I was right with my initial hypothesis. It was indeed the heat sink.

I was mislead for months because the heat sink I purchased initially to test my theory was also malfunctioning in the exact same way. But after isolating literally EVERY part in the iMac, buying a new logic board, and purchasing 5 new graphics cards (without heat sinks) to no avail, I finally decided it must actually be the heat sink, and it absolutely is."