So you manually turn down the gain on the CSM gyro and it still twitches, but it does not twitch with the gyro physically out of the circuit.

This is not logical, if the gyro gain is turned down it should act like it isn't there and the system should behave as if there was no gyro.

I am sorry, I am stumped.
I still would look for play in the tail pitch system, or binding there.

One other thing--swap channels on the radio, just temporarily. That is, put the rudder on a different channel in the TX and the RX. See what happens. I know it isn't logical but neither is your gyro result.

Two more other things--I have gone back thru the thread and I can't find where you may have tried the gyro reversing switch.. also where are you mounting the gyro?