I doubt it is the PSU. I have a SeaSonic - PRIME Gold 1200W 80+ Gold Certified. My graphics cards are Radeon Pro Duo Professional with 32 GB memory. Their consumption is 200 w maximum while mining.
your cards can draw more than 250watts/cards during mining ! Max Power Consumption for those cards during "normal workload is : 250W (Typical Board Power (TBP))
I cannot even get to mining. I cannot even switch on the computer. When I turn it on it freezes with a black screen and it shows nothing. I tried 2 psu. I changed the risers, the same thing happens. I think it something from bios but don't know what for sure.
The gpu are dual so they are seen as 2. When I insert 4, motherboard sees it as 8.maybe this is the problem