Check your "phasing" or timing of the controls. These may be off. When the head is parallel to the fuselage, the inner and outer rings of the swashplate should be inline. Also make sure the washout arms are parallel to the rotor head as well.

Also check your trim in FFF. If the model is heading down when in FFF, that will kick up the nose when you roll inverted.

You may also need to add some back cyclic during the roll as you may inadvertently be pushing forward on the cyclic stick when you start the roll.

Jeff