This is suited more for Nvidia GPU's rather than AMD. It may be that your R 390x's are just too outdated (Please correct me if I am wrong).
People have also reported that their RX470/480 is only giving them 5 -10mh per card.
Might be worth sticking to Ether or something else that uses suitable algo.
It's just Nvidia miner developers are more active. I wish some pro AMD miner will take a look at the miner and fix it. I will even ready to pay a fee, just let my old AMD card hash signatum
