When you try this solution, please make sure you are stand behide the helicopter, this is what we do when we are preparing this document.

Also, when you adjust the "GAIN" switch on the 4-IN-1 MainBoard, please disconnect the battery from the helicopter first before make the adjustment "GAIN" switch on the 4-IN-1 MainBoard.

If you adjust the "GAIN" Switch on the 4-IN-1 Mainboard to the right direction as the arrow shown in the above picture, the tail will move to right direction as shown in the second picture above
If you adjust the "GAIN" Switch on the 4-IN-1 Mainboard to the left direction as the arrow shown in the above picture, the tail will move to left direction as shown in the second picture above

So the only thing for you to fix the tail moving problem is to find a position of the GAIN switch which will balance out LEFT / RIGHT movement, then you will have a stable tail. The GAIN position is depend on the helicopter, and each helicopter may have a different perfect position.