2007 v50 passenger side mirror shake

[URDRWHO]

On our new to us 2007 V50 AWD started to exhibit mirror shake on the passengers side mirror. It started on its first long trip and was at highway speeds. I was afraid the darn thing was going to fly away. But it didn't.

We arrived at our destination, did some driving around the town but mostly walked. Then we left for home on a windy, rainy day. No mirror shake the entire 150 mile trip.

On the way down I was thinking that it was glue that hold the mirror onto the stalk (is there a better name). But then when it cured itself, I started thinking is this a mechanical / electrical issue. Maybe the entire stalk gets loose because it isn't getting the correct electrical impulse to hold it in place.

ANy ideas???