Allow me to answer my own question. Out of curiosity, I pulled the main rotor blades and bench hung a Futaba S3004 servo plugged into the ch. 2 elev slot on the RX. When I spun the motor up, all servos function as advertised. I then pulled the digital servo out of the frame. In looking closely, I found that the white signal wire had been chafed through the insulation, and had been contacting/grounding out on the carbon fiber frame. This was covered up by the tywrap that secured the servo lead to the frame and away from moving parts. It is my theory that the RF noise created by the motor fed directly into the servo signal wire via the carbon fiber frame, thereby rendering it inoperative until the noise/interference ceased. At least that's my story and I'm sticking to it.