Rig: 6xRX480 Sapphire Nitro +OC 8GB, Windows 10.

I use HWINFO64 to inspect the memory errors reported by each card, and so should you.
The rig has been mostly stable 175-180Mh/s for the last six months or so using Jukebox v4.rom bios on most and vdrop+.rom on one.

Recently I have had to lower the memory overclock and bump up the voltage 6.25mv on all my cards since Epoch #130. (WattTool -16 to -15)

I think what is happening is we're using more of the RAM in the new epochs so the bleeding-edge overclock that was stable before no longer isn't.

So since these changes we're stable again, but with 168Mh/s instead of 178Mh/s.