The hotter plug idea worked for my GMS 1.20. I had run about a gallon of fuel through it, but it continued to have a really rough midrange. Idle was fine and full throttle was smooth, but when I'd notch it down to half, it would slowly drop revs and start smoking and sputtering. It never quit and gave me a dead stick, but I wasn't too happy with that. An old pro suggested I go to a hotter plug. One of the guys had an O.S. #6 (old A3) and we swapped it out. It changed everything: the motor started running smoothly from idle to full throttle. At midrange, it was smooth with no sag or sputter, and transitions were effortless. I took the plane up and the engine performed beautifully. Give it a shot...