Upgraded iMac 5k restarts randomly
Problem solved. I believe the 1.5mm thermal pads were too thick on the GPU rams and the cooler didn't make enough contacts on the GPU chip. I went to an Apple service provider and asked them nicely if they could sell me the thermal paste they use and they were very kind and gave me a bunch of 2CL syringes with paste manufactured by Shin Etsu Microsi - microsi.com, that is what they use on apple products and it did the job.

I put a small pea shape bit of paste in the middle of the GPU memory chips, they are 8 and I used 1CL for every chip and when you put the cooler ontop it spreads it evenly. You could use less and it won't leak when you push the cooler in but i wanted to be safe and fill the gap between the cooler and the GPU chips.

Be careful, this paste is thick, its a pain to spread it evenly. You have to get a credit card and put paste on the top edge and swipe it with a bit of force so you can apply it in one take because its a mess to spread it with your finger, its like it won't stick to the chip.

I now have 56 Degrees on the CPU and 55 degrees C on the GPU die, lightly used and never went over 75 degrees C no matter the stress of the system. Measured with iStat Pro.

I hope it will help someone even though nobody cared to give me a hand.

Peace!