I read in another thread that the servos you used on ailerons was Futaba S9252. I also installed that servos on the ailerons, but there seems that these servos in conjunction with the supplied linkage and the weight of the ailerons all together forms an oscillating loop. Both my ailerons would "flutter" when the plane stands still. I have not tried to fly with these servos so I don't know how this would affect any real flutter once airborne. I don't know if this can have something to do with your flutter, but it may have some influence. Maybe you where very near the "natural" oscillation frequency (as I suspect your where as mine where spot-on with the same setup as yours), and that this triggered the flutter in air.

Changed to S9451 on the ailerons and they are rock solid without any oscillation tendency (yeah, I know they are a bit overkill for this plane...but they work!)

I'm using the 9252s in the elevators though and they do not seem to have same problem. I can see a slight oscillation there also, but only for a fraction of a second if the servos is stopped really quick.

Are you using the 9252s on elevator? I'm getting a little nervous about using this servos now.

Regards
OAK