I would check the muffler pressure lien. Pinhole leak, carbon or tar partially blocking the muffler vent, etc. Also is the engine broken in? Sometimes engines will not draw fuel well at mid range, not as much muffer pressure, and not as much carb suction as at idle, so it is in the poorest fuel draw at midrange. So you have it set slightly rich at full and midrange, it leans out at midrange, speeds up, gets more muffler pressure, then gets a slug of fuel from the muffler pressure, runs rich, slows down, muffler pressure drops, leans out, speeds up, then keeps repeating this cycle. Sometimes just more run time improves fuel draw and helps greatly.