Here is an approach to diagnosing the flutter. Turn the radio on. Grab the aileron at the trailing edge closest to the aileron control horn and try to deflect the aileron. Observe what allows the aileron to deflect. Is it the wood of the aileron bending, the hinges giving, the control horn mounting, the pushrod, the servo gears or the servo mounting? Then grab the aileron near the tip and try to deflect it. Does the aileron twist before anything else gives? Correct the elastic part by making it stiffer. If the servo output is rotating under load, then a better (possibly digital) servo is needed.

BTW, although Monocoat is not the easiest to apply over solid balsa, it is far stiffer than Ultracoat. One ounce per square yard kevlar in laminating epoxy, with the threads running at 45 degrees to the length of the aileron, will impart more stiffness for weight than any other covering. It is necessary to completely encapsulate the wood to get the best results.