Seat Belt Locks Up

On the passenger side middle row seat, in my 2004 Sienna, the seat belt locks and you can not move around. It is as though you moved forward very quickly which is suppose to lock the belt, but this happens when making casual movements. This happens only when the car is moving, and generally stays locked until the vehicle stops moving. Once you take off again, the problem happens again. The dealer has replaced the the seat belt assembly, under warranty, but the problem persists. Now they say that the new cars in the showroom exhibit the same action and therefore there is nothing they can do. Has anyone else had this kind of problem on their Sienna? Were you able to get it resolved? My position is that just because it happens on other cars in the showroom doesn't mean that I should accept it.