I agree with Eric that it is the push rod. My Focus II with O.S. 140RX in Hyde ARIAS mount had the flutter as well.

Option 1. Toss the central hobbies push rod in favor of a Dave Brown push rod (make sure you get the long one). Install per instructions but add foam blocks in at least two places down the fuselage for the rod to pass through. If you use the Dave Brown rod you will have to enlarge the hole in the rear fuselage to the size of the push rod - I found some long brass tubing in a hobby store and sharpened one end, took some time but it cut through.

Option 2. Install JR DS368 digital ultra-torque micro servos - one in each stab and use a JR match box to get them moving in synch and the same direction. You will also need an JR 36" extension. (I'm thinking about doing this but don't want to weaken the stab structure in the process)

Eric Henderson has alot of experience with the Focus both I and II, his advice is good - although I never saw any mention of neccessary support of the rod in my instructions, and I just set them down.

Alot of airplanes have their own "quirks", this one belongs to the Focus II.