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))