My Rx 6800 clocked at 72 Mh / s when mining Zil and back to Eth they clocked at 65 Mh / s but the rigs were crashing.
Did you enable compute mode?
This mode is not available on newer AMD cards. Only the old ones have this option, RX570, RX470 and so on. But since vega56/vega64 this option is not available.