_kaurus can you check if your HDD or SSD is partitioned with MBR or GPT? MBR does not support Above 4G Decoding, but the symptom should be system lockup.
Other then that I would remove one of the 1070's to see if the R9 390 got recognized. Or check if the system will accept 4 or more 1070's if you have more.
I'm using a MBR partitioned ssd with win7 and it works with 7 nvidia gpus. On the asus prime z270-p offcourse.