Seal the gap between the flaps/elevators and the wing/stab?
Well I usually do it by using hinges, Z type, the entire length of the flap and elevators. But on my 585 Stiletto, I used standard hinges. It was not all that stable in level flight, it did the usual slight but noticeable hunting. The maneuvers seemed too big as well.
Did all the usual trim tricks, line rake, line length, handle, etc. Still not quite happy. Then I realized the control gap was the problem. Sealed the gap with tape, flew two great flights yesterday.
Oh, yes. SEAL the gaps. You will NOT be sorry.