Hello Paul.
If I may suggest you try the transmitter with a separate receiver and servos on the bench with the same crystals and see if it still twitches.
Also
If you have another multi channel transmitter try that with the set up you have in the boat and see if its ok.
If its ok in the boat with a different t/mitter and the one twitches on the bench then it may well be a duff t/mitter...
I have just had the same probs with a transmitter, brand new and was ok for a short while ( 2 wks ) then it suddenly started to twitch ...badly... tried an old transmitter and everything was ok. Tried the new one and disaster, it went totally crazy as soon as you touched the thing, thank the lord I was only bench testing, returned to the model shop and they confirmed it was probably a dry joint somewhere they said........new transmitter given and all ok again.
Has the pup dried out yet ?
Sub.