Try to set 840 for example and tell us if it works, cause it looks like I had the same issue. I mean check wich card is showing you wrong voltage and set separatelyfor each card with comma, like: 840,870,870,870,870
--- Update: I've set 848 value only and it works for me, try to decrease the value. Also it look's like the problem with values setting is there and you need to restart the miner every time Fixed: too much undervolt