Question GPU turns off after some time

Sep 20, 2020
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Hello,
today I uprgraded my PSU in my Computer, so i have enough power, when I get an Rtx 3080.
After completly installing the new PSU from NZXT with 850 Watt the computer boots completly normal.
But after some time maybe 1 minute, or after starting some which needs many graphical Power the GPU turns off and i'll get a black screen.
I checkt if every cable is right connected but nothing works.
I have no plan how to fix it.

Mainboard asus z97 pro gamer
CPU i7 4790k
GPU: gtx 970 from asus
PSU: NZXT c850
 

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