Why is my tail servo doing this?

[JoshNZ]

This is with my gyro set to rate mode, -30 gain on Taranis, so ~65% gain in rate mode, but same symptoms with less.

The first half of the video, the twitching, is with a very slight bit of rudder input, either direction, and just held there. The second half is slow movements from centre stick to one side - notice the servo goes the opposite way for a brief moment before turning the way it's supposed to.
Same happens with TX far away, it's not because I'm close.

In flight it does the same thing, slight rudder left turns the helicopter slightly right before it turns it left, and is unstable when holding a slight bit of rudder. Swinging the tail back and forth on the bench lets the servo move through its motions smoothly so I don't think it's the servo.

I connected a cyclic servo to the gyro and it does the same thing.
It's a stock blade 500 so G210 gyro and Spektrum S400G on the tail.

Any ideas...? Can't really afford to replace it at this moment.

[phoenixheli]

It could be the gain, loose wires, gyro settings, the gyro itself, other radio settings, worn wires or even the rx itself.