My computer suddenly started to power off after about two hours of mining. Start back up it works ok for a couple hours then same thing.

 I've dialed back the core to zero and memory is at -400.  TDP is at 60 on one card 80 on the other.  Temps are fine.  Ccminer mining zcoin.

Power supply or motherboard most likely?  Seasonic platinum 850w. Motherboard is a msi tomahawk b350. The two 1080ti are not in sli.