The condition you are describing might be the result of wash out. Wash out is a slight twisting of the wing to lower the angle of attack at the wing tips. This is done so the center of the wing stalls before the tips resulting in a nice friendly nose first stall. When inverted, the effect is just the opposite forcing the wing tips to stall first, and unless they stall absolutely simultaneously you will get a snap roll. What plane are you flying? Many sport planes and early pattern planes use washout.