I have two of the OS 5AX motors and have quickly decided they are my favorite two stroke ever. My first one I bought for a 3D plane and I noticed the same thing about the mid range. It was a bit sluggish in the mid range with the bigger props. I had a 13x4W APC on it when it was brand new. Stepped it down to the 12x4 APC sport and it came to life. Now that both of mine are broken in I have no problems with hesitation. By the way a 11x6 is also a great sport prop for this engine. I would very much consider taking the Carb apart and making sure there is no debri inside any of the jets. Then go back and follow the advice of some of the other posters here. Very small increments on the low end needle should help you solve your problem. I also don't agree with the LHS owner. Your engine probably just needs some more fuel through and some TLC in the tuning.