That doesnt sound like a game related issue, so it has to be a configuration from outside.
We can try some options but i suppose AMDs driver software is capping the frames. Check AMD Adrenalin Software, since it could have a global frame rate control. AMD Software > Gaming tab > Graphics settings and look for "Radeon Chill", "Frame Rate Target Control" (FRTC), or a "Global Graphics" profile. Disable Radeon Chill and set FRTC to "Off". These are often the cause. Hope this helps you out.