Originally Posted by ricford
My guess is that there is a vibration from the drive. The whole boat is vibrating, it just shows in the mirror. Check your prop for nicks, check shaft alignment, etc.
I agree. Once on my 99 190 the mirror started vibrating badly. Ended up we had ever so slightly bent the prop on some sandy bottom. It was a very minor grounding so I didn't think anything of it. But it showed up in the mirror...