That sounds like bad Bios timings. It could also be your -dcri setting. I used 6 or 7 for my RX 580's and 5 for RX 570's. But yes, dual mining profitably is dead for most, at least for now.
I tried stock, slightly modified and one click. Rx580s with dcri at 6. Claymore v11.5 without oldkernels. Too much fluctuation on the old kernels for me.
Anyways back to single mining. Starting to see some pretty good rx580 prices on eBay but I wish it would drop more.