Your thoughts on flutter are dead on, it will most often occur when the surface is at center and not being "used" per say.

I would say the 1/4 inch of flapping is something in your linkage or setup, it is not in the servo. Even when they are worn out you won't see movement like that. Something else is wrong. To ge this much movement on a surface I would need to rotate my servo close to 5 - 7 degrees. You will see a slight amount of movement at the trailing edge but it should be minimal. For example on a KingCat elevator which is close to 3.5 inches wide, I have about .050" of movement after 1150 flights with 8611A's in place. They were rebuilt when the model was repainted about 200 flights ago with new gear sets, but that is all. The difference noticed was nothing from before I had them rebuilt to when they were rebuilt.

Show me apicture of what you have and maybe we can find an answer to your situation.