I had this exact same problem several years ago. All was fine for a long time, then all at once there was lots of servo twitch and zero range with the motor running. I chased it till I was nearly blue in the face. Finally found a radio switch that was worn and the vibration from the running engine caused it to have intemittant contact. Replaced the switch and the problems went away. I just plugged the battery direct to the reciever to find the problem.
It could also be an inteference problem as stated above, but check for a loose or worn switch or connector first. Epecially if you have no range with your antenna up.