Sounds like the card is the one being triggerd. Time to check the card out in another pc.

I tested a lower power target and lower clocks, both to no avail. Afterwards I replugged some cables in order to test for problems with some connectors. I will report if this fixed anything. Next step for me would be trying to use the default Seasonic PCI-E cables, to rule out the CableMod cables as a source for the blackscreens.

So just a quick update: I replaced the PSU with the same model and this unit shuts down the card too after a while -> it was definitely not a problem with the unit. For testing, I will replace this new unit soon with the 1000w model because I measured the power draw quiet sharp - even Seasonic recommends to buy at least the 850W version. If this one fails too, I will have to RMA the card probably, since I'm running out of sources of error.
What I observed so far: for changing the PSU, I removed the side panels. This results in an overall temperature drop of the system by 5-7 degrees, depending on load of the CPU (custom waterloop for CPU/MB VRMs/GPU). Since the panels are missing, I did not get a shutdown of the card - after putting them back on, it will take 1,5 ~ 2 hours of gaming and a shutdown will happen. When my card shuts down, it normaly reaches about 70-73 degrees C, which is "okay" in my opinion. These temps shouldn't trigger a shutdown by the card itself or am I wrong here?
The whole problem seems at least a bit temperature related: when the system is running for quite some time and the card experiences a shutdown, the next shutdown will come after 30-45 minutes and I believe this is due to a warmed up case/CPU/MB/GPU.
Well, when the card gets hotter in the closed case, the PSU obviously would get hotter too. Could the combination of both result in a shutdown of the card? I don't have any knowledge concerning electricity and how temperature affects computer components, that's why I'm asking.
 
I will continue to test with the open case and also with the 1000W PSU which should arrive soon. I will greatly appreciate any feedback on this topic.

Oh wow, I wasn't expecting something like this. Thanks for sharing this odyssey. Seems to be a related issue, but your system completely shuts of, mine is just triggering a power cutoff through the PCI-E supply.
I will check if the 1000w PSU will fail too, if it doesn't I'll probably keep it, even if I think it is oversized for my type of system. However I payed a lot for the custom cables and I'm not willing to sell them and buy new ones - all in all it is probably cheaper that way. However, I will report whats happening on my side. Thank you for your feedback!

Yeah - you had the same issue as me for sure, my card would get the issue at 75c as well as 88c...
 
My replacement card has yet to go over 83c in anything and is running great. Really odd issue.

tmmy34
Hello. I bought that PSU. I will be have not problems?
 
https://www.x-kom.pl/p/399265-zasilacz-do-komputera-seasonic-prime-ultra-850w-80-plus-titanium.html
 

Card will be 2080 Ti

You should be fine..

Hey so keep that PSU? Tommorow will GPU arrive. Today PSU arrived.

@tmmy34
 
Keep the psu. All psu's degrade over time. Sometimes you'll just get a bad one. Nothing to worry about.

tmmy34
Hey guys. New update. My friend mounted all ,he know what he doing + little stress hehe. And all fine. Firestrike stress test passed 98,3%. No issues with new psu and new gpu :)  

 
Game On