I recently purchase a H9 PNP 1/4 Scale J3 Cub and have about thirty (30) flights on the airframe with no problems until today. The plane came equipped with a Zenoah 26 cc gas engine CDI Ignition. I added an Rcexl gas engine kill switch v1.3 upon receiving the airframe. Today the plane flew erratically and it displayed the typical radio interference problems to be associated while using a FM PCM receiver. I throttled back and regained full control and landed. I proceeded to do a range check with the engine off and transmitter antenna removed and got 120 paces before loss of signal. Next I performed the same range test with the engine running and got only 15 feet of range. I have a problem so where should I start? IMO it is ignition related and I'm new to gas engines.