The first thing I would check is the filter screen in the carb. If it is surging in flight at half throttle sounds like it's starving for fuel. If the screen is OK it could just be rich on the botton end. I usually like to lean the low needle until I notice a bog in transition then richen just enough for the bog to dissapear. This should be done at operating temp wich means when cold it may bog or hesitate until the temp comes up to normal