Hey there 
I have some Nitro+ RX 570s 4gb too. Have you ran using the ATI Pixel Clock Patcher after flashing the bios? (heres the link -> https://www.monitortests.com/forum/Thread-AMD-ATI-Pixel-Clock-Patcher). Heres why you need to patch the bios https://www.reddit.com/r/EtherMining/comments/6mcwdm/noob_question_amdati_pixel_clock_patcher/dk0ojvt/?st=jbdqcjjx&sh=27faa4b7. Also have you disabled Windows from auto updating drivers, thats given me problems in the past. I'm getting 28mh/s ETH while dual mining on my rx 570s rn so hope you get yours working
I have some Nitro+ RX 570s 4gb too. Have you ran using the ATI Pixel Clock Patcher after flashing the bios? (heres the link -> https://www.monitortests.com/forum/Thread-AMD-ATI-Pixel-Clock-Patcher). Heres why you need to patch the bios https://www.reddit.com/r/EtherMining/comments/6mcwdm/noob_question_amdati_pixel_clock_patcher/dk0ojvt/?st=jbdqcjjx&sh=27faa4b7. Also have you disabled Windows from auto updating drivers, thats given me problems in the past. I'm getting 28mh/s ETH while dual mining on my rx 570s rn so hope you get yours working