Check this link - I bet its the same problem you have:

http://www.xoutpost.com/bmw-sav-foru...m-stumped.html

As I noted in my post on page 3, the fault was in the relay switches of the general module. Scott Johnson (details in the link) was able to replace the relays for me and I've been good ever since.

Of course, before going into the general module, first make sure it's not the switch, regulator, or motor. But intermittent failure makes me suspicious of the relays.

Good luck.