I've lost a plane to flutter that was a chronic problem with it.

The older Funtana 90 ARFs were known to have problems with the wings breaking if you flew too fast. I think they have fixed that in the last year or so. Funtanas do have critical speed limits.

The problem with gaps isn't the gap itself in regards to flutter. It is a combination of gap and uneven control surfaces such as an elevator installed with one side higher than the other. This means that you have an uneven amount of that surface up into the air stream and the imbalance starts the fluttering. Sealing the gaps does give a smoother flying plane.

Control linkage slop may be where your problem lies. My plane only had elevator flutter when it was balanced correctly. Having it nose heavy would stop the problem, but the plane flew badly then. How fast are you going on those downlines? Are your push rods stiff and secure?