Are you using Claymore's Dual miner? What is your intensity set at for Sia? I had to drop my -dcri to 12 to get back to good Eth hashrates. On my Gigabyte Aorus RX 580, I'm at 29MH/s for Eth and 350MH/s for Sia.
Is that with 1500 timings? I can only manage to get 27/600 eth/sc from my gigabyte RX580. They'll only run with the 1750 timings, crashes with the 1500. Dropping SC intensity does not take it any higher.