Sorry, I forgot to post an update. My problem turned out to be loose solder joints on a relay in the LKM (light control module). I forget where I saw the fix action (with pics and all), but there's a good description of the repair procedure at http://www.bmwe34.net/E34main/Mainte...trical/LKM.htm
When I pulled the cover off the LKM, the loose solder joints on the relay in question were obvious. I touched-up all the joints on that particular relay with a solder iron (loose or not), and the problem hasn't happened since. That was about two months ago.