You could try to add a tiny bit of up in each aileron. THat should help alot.

Mine does the same thing if I pullback hard & fast. If you slow down the initial pull a little it seems to help. I dont have full deflection in my elevators till the bird is pointed straight up. It still
does a 15ft. inside loop. When i changed to a 4-cycle for power
it also helped. Simply because the elevator pushrod now has a
straighter path.
P.S. I do use 20% expo on the elevator.
later, daveo