Futaba 9C Range Problems...
Without being there to look at your install I would first check the obvious. Does the glitching occur with the engine running as well as off? I'm assuming you are running a gas engine? Did you check for any metal to metal connections or other ignition related problems? I know you have set up quite a few gassers but it is possible to miss something. If the problems still persists did you check for possible rf noise from long extensions..etc? You said the reception seemed better with the antenna outside the plane. Have you had this receiver mounted in any other planes? Did you try using another receiver? Could your transmitter have been dropped or damaged? Check to see if your transmitter antenna is connected properly. I would agree with you that 50 feet is not an adequate range check, 150 is more acceptable. If another receiver/transmitter exhibits the same problem then you may be forced to send the transmitter in for service.