Spirit System

have you checked this page ?

http://manual.spirit-system.com/index.p ... Servo_list
>> For optimal performance of the rudder, we recommend to set the Rudder Delay parameter according the table. <<

If the tail wag is actually tail hunting - the boom is moving from side to side slowly then the problem can be too high Rudder Delay.
But in the most cases it is unfortunately a tighter mechanics.

The smoother the mechanics, the less apparent the effect of wind.

as ZeXx86 suggest check also the tail mechanic
disconnect the tail rod from the servo and try moving it with you hand
the mechanic have to move freely and smoothly