Is your card the 390x? I think you should be using the amdgpu instead of radeon.
Edit: https://wiki.archlinux.org/index.php/AMDGPU#Selecting_the_right_driver
Edit2: If you read through all this.
R9 390 series poor performance and/or instability
If you experience issues [5] with a AMD R9 390 series graphics card, set radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1 amdgpu.dc=1 as kernel parameters to force the use of amdgpu driver instead of radeon.
If it still does not work, try disabling DPM, by setting the kernel parameters to: radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1