Front Seat Belt Anchorage problems of the 2000 Lexus RX300

Since I have had the car (2/2000) the driver's seat belt has intermittently loosened 4-6 inches during normal driving conditions as well as while braking. The seatbelt has been replaced 2x, but the problem persisted and the dealer was unable to determine the cause of the seatbelt loosening. The factory rep sent out by Lexus also could not find anything the matter. The problem was always intermittent. In late June I rearended 1 possibly 2 cars while going at approx 35mph. The seatbelt failed to hold me in place, I hit the steering wheel sustaining an acute chest wall contusion (documented after being taken by ambulance to the nearest emergency room). Lexus continues to claim there is nothing wrong with the seatbelt.