Sounds right to me, maybe it's the same thing but the signal from the radio is a square wave or some such thing, and it's the length of the wave that is read and tells the servo to turn one direction or the other, so maybe it's the same as if the power source is weak. I do know that the radio antenna can affect it quite a bit, run the antenna along the servo wires and you get much chatter, but pull it away, in a different direction and that diminishes the chatter.
I'm not sure with this small plastic model what will happen with the antenna, there sure won't be room to stretch it out full length inside, especially not away from the servo leads, it's going to be a bit massive to have outboard. I know antenna length is important for good radio operation, I guess I'll find out how important. I'll have to experiment (it will be tough to take the nippers to the antenna on a new micro reciever but what the heck), maybe a single strand of uninsulated wire from the bow over the mast to the stern? Maybe trail a thin wire in the water? Probably get caught in the propellers, we'll see. Welcome any suggestions on that (the boat is only about 13" long). Cap