ibew595 seems to know a lot about the door locks and I'm sure he'll look in on you shortly. Apparently there's some plastic parts that are very subject to failure.
Johan & Sean's site has several doorlock items. Their cars are E32s, a little older than yours but the doorlocks are probably very similar if not identical. The link is here:
http://bmwe32.student.utwente.nl/
Also, there might be some info here:
http://www.e38.org/
Go ahead and take off the door trim and watch the action-the problem may become obvious.