Part of SRS that's a step up from the older RS is seatbelt pretensioners. Under severe deceleration such as on impact, an explosive charge will reel in the slack and lock the seatbelts. Once this happens, the seatbelt assembly has to be replaced.

My guess is the pendulum mechanism that temporarily locks the belts around turns or while decelerating is sticking and temporarily locking the belts. This might also occur when the car is on an incline.

Sixto
87 300D