Optiminer has never been stable on my rigs.  They are 6x rx470s on ubuntu 16.04 w/ amdgpu-pro 16.40 drivers.  I've tried less aggressive bios settings, and running lowest intensity...    I really need to break it down to 1 card at a time, but just havent had the time.

Also, I can run ethdcrminer64 pushing 1030w all day every day...  so I highly doubt it has anything to do with my risers getting hot (which I read fixed it for someone on the zcash forums).   I also have multiple rigs, some with v3 risers, some with v5 risers, etc.  The only constant is ubuntu 16.04 and amd driver versions.   Also sgminer-gm works for days on XMR without issue.

Someday (when I have to) I'll figure this out.