It's been my experience that usually something like this being caused by what I refer to as " interior RF " in other words there is something inside the airplane that is causing the problem. Being an electric model add slightly to the mix. Are you using the BEC in the speed control or a separate RX battery? Is the RX mounted closely to the speed control or motor batteries? What brand controller do you have? If you are running BEC then I would suggest installing a separate RX battery and testing again with the FM RX. If there is a problem with the BEC introducing dirty voltage to the RX it may get worse and the SPCM RX may have difficulties rejecting the noise at an unpredictable time.