My thought is that if you felt it in the stick and it took a tight grip to control it it is likely flutter. As others have suggested, check all control connections, security of the mass balance weights including the flaperon wing attachment fittings. I have flown lots of hours in some pretty rough mountain conditions, and I don't recall any wing movement that affected the stick.

Regarding the Kitfox and stability in rough air. I think it relates to wing loading. The Kitfox can get pretty bouncy as they are light, especially the earlier models. keep in mind, though, there have been no reports of inflight structural failure. That is what keeps me going.

Lowell