Tip down mirror gets tipsy

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?
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My folk's did the same... it dutifully tips down... but never come up... turns out they replaced the motor to fix it.
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