How old is your engine? If the rubber inside the spark plug boot is black, it is an old ignition system that gives lots of trouble. I had one which ran fine on the ground but when flying would cut out intermitently. Steve Chiao at Cermark replaced it with a new unit (gray material inside plug boot) free of charge. Have several flights with new ignition, no problems, engine (SPE 26) running great. Plenty of power for my purpose, 18-6 prop at 6,600 rpm. I have it mounted on a Great Planes Super Stearman. He claims the new ignition system has solved the problems they were having with the old one.