Less than smooth running in the midrange is very common in model engines. That's because the carburetors are not linear. If the top-end and idle mixtures are set correctly, and the engine doesn't quit when run for extended time in the midrange and also has a good throttle-up and throttle-down response, then don't worry about the midrange.

You may need to check your high-speed needle setting. It's possible that it's a bit rich, yet. We usually use a tachometer to set the needle when we're running large props.