One rig of a friend suddenly start mining at 50 MHz/s with one RX580 8Gb. And still running 4 hours stable.
The "X files" RIG is named "RIG4-R390"... now mining at 74 Mh/s !!
your card never mined 50-70 or 74 MH/s , not even close to that.
your mining software have a false reading for some reason, that reading sent sent to the pool , as a reported hashrate
you real hashrate is 15.1MH/s
thats the worst i ever seen for a RX580 card 
