Maybe cleaning and reseating the card was sufficient. Do NOT go further if it works now, replacing the thermal paste, changing the cooling solution, is a good way of killing a video card easily if you make a mistake in the process.
Thank you for your concern. I was careful and looked up an tutorial. I disconnected the cool segment carefully and disconnected the fan power supply cable. I didint touch the cooling pads and used propanol to clean the chip. I cleaned the Heatpipes as well. After this step I started with the application of thermal paste. I made sure to put not too much of it on the unit.
After this step I build the card back together.
First start up: Same resoultion problem again both systems. The temperature of the card was ~10°C cooler in idle ( 45°C before 32°C after).
Then I reseted my mainboard bios to check if it helps.
Start up again everthing normal card still cooler.
Intersesting is SAM is now off. Maybe this is the orgin of the problem. I will do further testing later on.
Update: After getting Home, same problem with blackscreen on startup :(. Restart solved it.
Temps are higher now 50 °C in idle. Put the GPu under a stresstest ( shooter with wqhd on and max settings): no problem at alle 100 fps Gpu temp 60 - 70°C Junction temp 95-100 °C no lags no artifacts.
Off topic but there is not really ‘too much’ paste issue possible, this is a myth, it just gets ‘dirty’ if you really put a lot (unless you have an electrically conductive thermal paste in which case you really don’t want to have paste elsewhere around the chip when it squirts with the pressure). Not enough thermal paste on the other hand is an issue 
Haha okay I will remember this, thank you for your help system seems to run stable now. At least it seem so.
Solution recapitulation:
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Replacement of thermal paste
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reset bios (maybe deactivating sam)
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cleaning and remounting GPU
If sth changes I will write again.
Oh no it aint. oozing out and creating shorts/rifts/blah is a real thing.
Never underestimate the overindulgence of end users 
Literally what I wrote. Most thermal pastes are not electrically conductive anyway. But regarding thermals, this is a myth, there is no too much paste regarding that.
May be they are not. But then you have ‘ooze’ over other components … and other things. Maybe I was just being too pedantic. It just seemed like you may have opened the whole concept of ‘there is never too much’ … and well, you know how people on the internet are (let alone all the droves that never even post).
That is also what I wrote lol did you read?
It doesn’t make any difference.
