Re: PhoenixMiner 5.1c: fastest Ethereum/Ethash miner with lowest devfee (Win/Linux)
I had almost the same problem (my last card wasn't mining full hash instead of your first) and I was crying like a baby here 
ONLY solution I found was to update driver to 20.7.1 or 20.8.1
ONLY solution I found was to update driver to 20.7.1 or 20.8.1
I have RX580 and updated to 20.8.1 and my last card is not mining since few days.
Not sure how to fix that for now, but at least the first 12 cars are back now working.
Looking for Linux and thiking if there is some reason to buy 8GB cards soon.