I dont know if it matters but I had the same issue with 2 of my RX580s ASUS cards. I own the Dual OC 8GB Edition. Both of the randomly dropped their hashrate. Sometimes right after the miner started, sometimes after a few hours of operating.. after they dropped their hashrate the whole rig rebooted. i had up to 200 reboots in 24 hours.
I did everything.. flashed back to original bios version, no OC, hash with only 1 card at a time, put them in 16x slot and connected HDMI monitor. Nothing worked..
Lucky for me was that the store i bought them from accepted the return of the 2 cards and I bought RX 570 4GB sapphire pulse. never had an issue afterwards..
btw I also changed risers and so on.. nothing worked at that time. now im using the same risers with my Pulse cards.. works fine..
if u can, return the ASUS cards or sell them for other cards..
I'm done with ASUS cards for mining.