I have problem with older Windows 7 rig. It has 2 RX 480 and 2 RX 580, all 8GB versions. They all mine just 14MH, on both Claymore and Phoenix miner.
Equihash mining is OK though. When i mined Ethash before it was about 25MH if i remember correctly. Cards are not flashed, slightly underclocked and undervolted.
I reinstalled drivers, ended up installing newest beta. Trying to figure it out for couple of days, probably going to reinstall Windows.
Any suggestions?
Have you switched GPU workload from Graphics to Compute in your drivers?
It could be the problem.
Or try installing Beta blockchain drivers
https://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-Crimson-ReLive-Edition-Beta-for-Blockchain-Compute-Release-Notes.aspx