I hear your pain on flutter. On my first Jackal, I had a flutter shatter my entire elevator control system, clevis, pushrods, servos, etc. Clearly, I lost the plane, with a high speed nose dive. The crash was spectacular.

On this new Jackal, I redid everything for strength and durability to reduce or eliminate flutter and give myself full redundancy through the entire system. For all control surfaces, I replaced every clevis and servo connection with 4/40 titanium turnbuckles capped with ball links. Servos were upgraded to all metal Hitec servos with metal horns. For the elevators, I am using 2x 236oz servos on 7.4v. One servo for each elevator half. On the Rudder, I am using a single 90oz servo. All this together, greatly reduced the slop in the surfaces and flutter is a thing of the past. I will take some pictures when I pull the elevator off for some routine maintenance before flying this weekend and post them here.

The other thing you can do is try pulling the rudder off and putting in a new pushrod that is a bit longer and redrill the hole a bit higher in fresh wood, just incase the hole got enlarged due to flexing. I hope that helps a bit.