Update on the problem.
I went to the pond this morning with a friend to follow derekwarner decoy's suggested tests. Results; radio connection is reliable and positive with the Tx antenna down to a distance of about 200 feet. Over that distance and the motor shut down, with the rudder holding its last position. This is the way it should work according to JR.
Next, I tried the puffer at a slower speed, about 2/3, to see if the twitch re-occurred. It did, but not as much as before and not as often.
As to the questions from Bunkerbarge; the motor is a Model Motors Direct 555 and the "full" speed is less than walking speed, perhaps 2 mph. Definitely not a hotrod! There is less than 1/32 rudder free play at the aft end furthest away from the hinge.
An idea came to mind, maybe completely wrong, but then I am usually so.
The pond is man made and about 20 inches deep. I know that there are many small rocks, about 4-5 inches round, here and there on the bottom. I wonder if the prop wash goes down, hits a rock and is deflected back up which affects the steered course just a little. Or the fact that the puffer is quite heavy with a very unstreamlined hull shape may make her very sensitive to water flow changes reflecting from the pond bottom. What think you?
If you tell me I'm waaaaay off base, I won't be offended.
Ian