What temperatures are you showing for your GPU's Hot Spot? is it at or above 110c?
Do you have an aggressive GPU Fan profile so that the fans runs at maximum speed at a lower temperature?
Does this happen only with one game or any time your GPU is under heavy loads or stress?
If it happens only on one game you might try different Game and Radeon Settings to see if it works better.
The reason I ask the above questions is to rule out overheating GPU that will throttle and lower your FPS.