I have only just picked up this thread but, If your car has automatic seatbelt tensioner,which most have and certainly mine does, which operates on air bag activation, just check your connectors below your seat, after a passionate encounter and my seats right back I had this problem, Ford wanted to replace the seat belt tensioner, all I did was pull the connector apart and then back together, never played up since, I really hope this sorts your problem.
Neil
