Ok so I guess it's fixed
I pulled all cards except the one I had plugged into the x16 slot (PCIE2). -- It crashed.
I unhooked that card and put another card into the x16 slot -- stable for 5 minutes!
I hooked the card that wouldn't work into PCIE1 (x1) along with leaving the working x16 slot card -- it works!
Now I'll hook up the other two cards and let it run overnight.
TL;DR
For some reason my x16 slot doesn't like one of my cards. It's not the riser/usb cable/etc. All the cards are running identical firmware and are the same exact card.
Cheers!