You might want to take it to an authorized mechanic and let them diagnose it. I found out that they have regulated pressure problems with the switch. It may be applying too much pressure to the clutches at certain times, and then not enough at others. Also, you may be having issues with the Pressure Valves inside the tranny. You might want to do a search on the problem and read through some common issues with that particular tranny. I have been on the internet trying to find information about the electrical system on my '88 735i. I have narrowed down my issue to an electrical short. The tranny is not getting any voltage to the switch. Hopefully easy fix. Sorry I don't have more possibilities to suggest. If I think of anything else, I will post.
Curtis