Got a rig with 6x1080ti, mining ZEC

Was doing fine for about 2 weeks but now GPU4 stops:

http://i.imgur.com/iXJoYLH.jpg


Watchdog restarts the mining process but GPU4 doesn't recover.

http://i.imgur.com/oorGsXW.jpg

Team viewer and miner freezes when I try to access the Guake terminal, but the rig is still accessible via SSH and seems like the RSS process also still works because the rig is not rebooted.

I already replaced raiser, didn't help.
I dropped OC down to ZERO both on GPU and Memory, didn't help.
I raised POWER from 180 to 230, no effect on stability.

Any ideas how to troubleshoot?