"...vertical dive..."
That sounds like you're holding elevator for level flight.
A trimmed plane can't know whether your fingers are on the sticks.
In the real world "stick-free" stability is a testing criteria, but with our models, the servo holds the surface so the plane has no genuine "stick-free" mode.
Are your ailerons aligned along the true chord line of the airfoil?
If the aileron trailing edge is down relative to the zero chord, it acts like a flap, which increases the nose-pitch.