From a month ago all R9 390's have hash rate 27Mh/s on Ethereum
I tried reinstall with win10 1709 and 15.12 amd drivers.... no change
When I switch to musiccoin everything is OK hash rate back to 31Mh/s
The only deference is EPOCH of the coins.
I have no idea Sad
Yes this is normal. I am getting the exact same issue with my R9 290. Basically this is similiar to the R9 280X DAG thrashing issue we all had back in 2016. With a new epoch the speed decreased more and more.
I've noticed my R9 290 were pretty stable until the last few months. Running at 947/1250 (stock clock) resulted in 26.5MH/s and now that same clock only provides 23.5MH/s. Every DAG I think it loses something like 0.1-0.2MH/s of speed.
Nothing you can do really. I doubt there will be a fix coming in from AMD since these are old GPUs.