I know this is an old post, but as this sounds like an auto gearbox problem most BMWs of a similar age have exactly the same gearbox! a ZF 6hp26 IIRC or the weaker ZF 6hp19. so just as likely to have the same problem.
When I have problems with my British or Italian cars the culprit often has a ZF, Bosch of Getrag label on it.