Surcace oscillation can be very dangerous, I have saw planes actually loose the affected panel.
Possible Causes:
Excessive Hinge Gap
Excessive "Slop" in the Push rod Connections
Control Horn Slack (Slop)
There were a lot of good post above on possible fixes.
I even use toothpicks while building in the hinges using soda & thin CA to reinforce and wires in the tail feathers with soda & thin CA to secure with the use of extra small diameter supports.
The covering will cause issues as well.
I was given a Tiger ARF FF ( had loose covering-I tightened it with a hairdryer).
For instance the Morris ARF was heavier than my kit version of the Knifes (2) I had built previously. I have built the Hotts-Knife-Profile Mustang all good planes.