Radeon VII used to do ~92 MH/s out of the box, now it barely reaches 80 MH/s.
Momentan solution is to restart miner every couple hours.
I faced same issue starting one month ago, it repeats 2-3 times every 24 hours, drops from 94,5MH each card to 78-82MH.
After 3 days of pissed off, I sold my entire rig based on 4x Radeon VII, and I bought 8x 5700XT to make a new rig, less power consumption, less heat dissipated, less noise, more MH, but more space too.
Much better efficiency for 5700XT, in my opinion, with modded bios id tbe best card for mining ETHASH.
https://imgur.com/zT2koazit was. Now rtx 3060TI best mining for eth. 63Mhs for 120w