Just a reply for anyone searching this thread someday. The problem was this:

When I assembled my tuner I was in the final stages of testing, and was SOO happy it was working (on the bench) that I slapped the cover on and installed it straight away. But... I forgot to put the SCREWS in the top circuit board that hold it to the standoffs. Eventually it worked loose (enough) that the tuner would no longer tune.

So, moral of story, if there are parts left over....