Dick, it's that sudden braking effect that seems to provide the Flat Foamies with their appeal for me. I had watched a lot of videos of the larger profile 3D'ers and the flat foamies and found that "something" was missing on the larger ones. Then it hit me. The FF's just STOP when you flick them up while the larger airfoiled 3D stuff tends to just loop very tightly unless care is taken to get it right.

Stek, re-reading your original post again I'm thinking the guys that are saying you may have a "wandering neutral" are right. Typically a 3D model is set with the CG right on the neutral point or super close to it. In fact many of the Flat foamies run the CG behind the neutral point by a little bit. But this all means that there is no built in pitch stability at all or, in the case of the CG behind the NP a tendency to diverge from the flight path.

For starters I'd use some form of indicator needle to check that the elevator returns to a postive nuetral within less than 1/64 at the trailing edge from both directions. I suspect you'll find that you don't have an accurate enough return to neutral.

From the sounds of it this happens at pretty much "regular" speeds so it's not that likely that you're getting any speed related flexing. But if your trim change shows up as the speed changes then it may be that you have some speed related twisting happening. Or you may have some speed related pitching variation happening due to being on the ragged edge of the CG placement.