Since your swapped your PSU,risers and motherboard it is save to say that the only thing left are the OS and Gpus
Check your Gpus on different slots and check which one stops working (might be only one gpu stops working on different slots.
If thats the case try cleaning the pcie x16 connector and try again.
I had a problem that form 3 of my 1080 tis ,one would stop between mining after 2-5 mins , I tried almost everything ,It would work fine solo but not when put in that rig with other cards So I pulled it out and put it back in 2 days later and to my surprise it started working (and now its still working since 2 months ago)
I am running simple mining OS on flash drive.....I have re-flashed it a couple times...do you think it could be a bad drive or bad flash causing this?