I've had the same probalem for the last couple years. 4 year old Cartels with Burton Rulers. Then I bought some Hail's thinking the boots were the problem but same issue. Replaced the ankle staps but same thing.
Finally replaced the Cartels with some Rome 390 bosses this year and havent had an issue at all since.
Not saying it's your bindings but maybe try a few runs with a friends bindings, worth a shot I guess.