Sometimes flutter is an aerodynamic process, not a structural one. What I mean is that even very strong structures can flutter under the right circumstances. You mention flutter "at speed", is it high speed in general or above a specific speed? What is the frequency of the flutter? That is, do you hear a loud buzz or can you see the horizontal tail flapping in the breeze? Does the airplane have stability augmentation? I have seem planes that had the gain on the gyros turned up too high and the tail wiggled like a tadpole. It looks like you are using stock hardware. Is there any give in the linkage at all? You don't mention what the elevator servo is. It's possible that it isn't quite up to the task. The FA 82 may just be a bit much for the airframe. I realize I'm tossing out a lot of possibilities and not much in the way of answers but often it is a process of elimination rather than having a answer right away.