I can't specifically help you, sorry. I do know through years of owning one of these, and several discussions with my favorite SA at the dealer, that the door hardware on the E30 is a known weak spot. Usually it's the cams and levers on the end of the lock cylinder that cause the problem. They loosen and wear. Sometimes they can be tightened, but for the best results, they should be replaced, including the lock cylinder in the worst case. safe to say, your problem isn't unusual. I had the dealer deal with the problem in my 325 and to the best of my recollection it wasn't ridiculously expensive. Under $150 IIRC.