I also have more than 10 pieces R9 390 8g and have a problem with hashrate mining ETH
Dudes read, it is a normal very old problem. Why so many people ask this question this days?

R9 290 and R9 390 have DAG issue. There is no fix for this, only mining Coins with lower DAG (CLO, ETHO)
I also have more than 10 pieces R9 390 8g and have a problem with hashrate mining ETH
claymore fix this problem with RX 580/570/480/470, but they said they have no support R9 390 8g for the moment
claymore fix this problem? XD no this problem where not fixed by claymore. It was a driver bug, and amd fixed this problem.