Yes, i'd guess that you have some difference in control linkages. The other possibility is that the CG is a little different but your chosen CG location method (end of fingers?) isnt sensitive enough to detect it. Incidence angle would not cause this, if all else was identical then diffrent incidence would just mean you had to trim the elevator, it would not effect stability.

Another option is that something is loose or flexing, maybe (esp. ifit's a foam plane) the wing or tail surfaces themselves are twisting, this can cause all sorts of erratic behavior.

Steve