Looks to me like it was the mechanic at the 100 hour who had changed
the vacuum filter that did the deed... I will remind him about that
next month when the annual is due...

That was the first thing that popped into my mind because we had the filter
changed at annual. I told my partner that it could be that simple and that
I would hate for them to "give up" and bring the plane home only to find it
such an easy fix. Our airplanes are getting pretty old and I've come to
expect the expensive and complicated rather than the cheap and easy!
It's nice to be surprised once in awhile!.

Jim