I had a similar problem on mine. Unhook your control rod from the servo and feel if there seems to be a very minor bind in the middle of the travel. This is caused by the eyelets that go from the slider and the blade grips being too tight and having a spring effect around center. Loosen the screws on the back a tiny bit on the eyelets and see if that solves the problem. I chased this for a week straight before I found the problem. It makes it hard for the servo to center when hovering and bounces back and forth over the "hump" in the travel....Thus creating a slow wag at any gain value.

I'm assuming you have the plastic tail slider, on the CNC slider, the eyelets are hinged and you shouldn't have that problem with it.