Ah, you found out why you need stiff ailerons, stiff hinges, stiff linkages, and counter balanced ailerons eh. If the previous statement were true a miniscule servo with no gear slop would be sufficient. Effectively by adding a larger servo, you are damping the system with its resistance due to its torque needed to move it off its assigned "spot".

To help eliminate flutter, do NOT place your servo linkage at the aerodynamic center of the aileron. If its a kit, most likely, they predetermined this for you already. Place it at a prime integer value of the AC. 1/3, 3/7, etc.

Enjoy your introduction into vibrations.

Cheers