CG sensitivity can sometimes change depending on how the wing is folded and glued, so the balance on the spar, although a good starting point, may not be perfect. Also, the ailerons have to be level with the fuselage or slightly reflexed. What you describe is directly related to aerodynamically drooping ailerons or tail heavy or a combo of both. Bring the ailerons up and try it, if that doesen't help, move the CG forward, and I'll bet you'll have a whole new airplane.