You''ll have to check the engine carefully to see if there''s something bent. If something was bent, though, you''d see high vibration levels at all RPM settings. Operation in the midrange can produce rougher running on some engines. It depends upon the propeller and the mixture setting. It may also be that the structure of the model really isn''t up to a large gasoline engine. This is more common that some may think...especially if the airplane''s being overpowered.