There is obviously some difference between airframes, some have this problem and some don't, all we are trying to do is suggest things to check, rather than boom stiffness its the play in the incidence rods that are screwed into the stub wing that cause the tail movement, it has been mentioned before to Hysol them into place especially if you are experiencing elevator problems, like flicking out and a change in trim before and after manouvres.

Mike