I will assume this is on desktop with all (non windows) apps disabled in "startup", no minimized apps/browsers, hw acc turned off.
There are some issues with some 300 series (may be related, being worked on). AB has also been mentioned to cause conflict with Wattman for some.
You could try a Clean Install AMD Graphics Drivers (the newer DDU in needed for Relive drivers), may/may not help.
I have Radeon Additional Settings but I don't have option in Radeon Additional Settings - DVI Settings <- that one.
Now its changed... I stuck on 150MHz and got artifacts(?) when for example I press "Refresh" from Quick Menu (right mouse click on desktop).
I can't change Memory Clock value in WattMan... Its really annoying... Thank you all for help. I will back to 16.11.5