Hi all.

Periodically the tip-down mirror function on my right-side mirror (99 E320)
gets a mind of its own.  While in reverse it dutifully aims to where it has
been programmed to point.  But after putting the car in drive sometimes the
mirror wanders skyward.  And it seems like the mirror motor is continuing to
push the mirror even after it has reached its maximum limit.  Perhaps it is
bored with the view behind me and wants to see the clouds, but I find this
quite annoying.  If I press the preset button it returns to its appropriate
position.  It does this only a few times a month.  Any ideas on how to
diagnose or some areas of probable cause?