I am running 5rigs with 4xgigabyte vega56 cards. GPU 920mv, memory 910mv @ 1100 Mhz. 7700 hash ( 2016/1736 ). All rigs are 20+ days stable aka nonstop.
I get those speeds, but not stable. What puzzles me is that cast-xmr works fine.
Are those flashed to 64 ? How are you undervolting ? I guess you're using blockchain drivers.