I saw the photos and as soon as I saw where you located the control horn clevis it screamed flutter to me. I've been building and flying r/c since 1974, control line before that. Why did you have the linkage so far in on the control surface?

Also,

I owned the earlier GP300s 40 size and had a Saito 91 in it. It was a great flying plane. The performance series is a lighter version with larger control surfaces.

I remember people complaining about the Funtana 90 fluttering when it first came out. Looking at the photos if the FUntana it was clear the plane had not been assembled properly and the wrong servos were used in the wings.