Are you certain it IS coming from the PSU, and not the motherboard?
Again I'll ask, is there any chance this power supply or motherboard seemed like they may have been returns that were shipped back out to you? Did ANYTHING look out of the ordinary in the packaging or arrangement of items inside any of the boxes? WHERE did you purchase them through, because I know for a fact that in some cases some online retailers will re-shrink wrap a returned item, that has not been tested, and send it back out.
At this point it might just be a very good idea if you are unable to test the PSU, to simply return it, get a replacement, and see if you have the same problem. If you do, we can go from there. If you haven't bothered to take any of the steps I've suggested such as completely removing the graphics card from the motherboard, entirely, and using the iGPU through the motherboard's outputs, it would be a good idea to do that because a problematic graphics card can cause such an issue. Also, you should really take another look to be sure you have EVERY connection correct. Make sure you have not tried to use one connector where another belongs (Common attempts at this include the 6+2 PCIe in place of the 4+4 EPS and visa versa) and that all connectors are actually FULLY seated, even if you think they already are.
A lot of people thought they had there 12vhpwr PCIe connectors fully seated too, but didn't, which was what was causing the majority of problems on the newer style cards.
View: https://www.youtube.com/watch?v=ig2px7ofKhQ