I have got a seagull pc9
i got snake rod through the plane to make the elevator more stable,i flew it on the weekend but i always got a flutter in the elevator coming down in a dive,so i put the clivis which i had on the very outer hole on the horn inside the plane , onto the iner hole closer to the screw that holds the horn onto the servo.
it fixed the problem.
it seems pc9 have a common thing with flutter.