My buddy had the same problem with his. The base of the fin is too weak. When they cut the hole in the bottom of the fin to mount the servo they neglected to add any reinforcing. You'll have to add some extra wood and epoxy around the servo and reinforce the base. It would probably be best to build a box around the servo.

Mine is an earlier kit with a thin wing servo mounted under a cover further up the fin and it doesn't have the problem. After Ron fixed his it was flutter free.

By the way from the pictures I saw posted of the new Liberty design, it looks like it will have the same problem.

Joe