Second PSU turns off??

Hey everyone,

My latest rig issue is the following...
After hours and hours of mining my rigs randomly have the 2nd PSU shut down and mining of course stops. Giant fans are blowing onto the rigs and are keeping fans running ~40% at 73-75C.

I have 2 280x cards on PSU#1 (including mobo/cpu/risers) and 3 280x cards on PSU#2. They're connected by a simple splitter cable.

I thought this has to do with overclocking or overheating so I measured the watt pull at the wall:

Cards - Sapphire DUAL-X 280x - Hynix memory - stock gpu = 1020Mhz
Power supplies - Corsair HX850 Gold+
Each rig on it's own 15A circuit!

PSU#1 - 888W (2 cards)
PSU#2 - 650W (3 cards)
Gpu - 1060Mhz
Voltage - 1.225v
Hash - 735kh/s
Temp - ~75C

i 13, TC 11200, gpu 1060, mem 1500

I then undervolted them and turned down the clock on them. I also moved two risers over to the second psu to balance it out more and here's my measurements:

PSU#1 - 645W (2 cards)
PSU#2 - 710W (3 cards)
Voltage - 1.150v
Hash - 720kh/s
Temp - ~75C

i 13, TC 11200, gpu 1040, mem 1500

And the issue STILL happened?! I'm running out of ideas and I'm completely stumped. Any ideas? This is happening on multiple rigs with the same setup.

Thanks,
llama