Obviously, I have to check "downstream" and make sure there are no
shorts.
But I was wondering whether these protectors ever fail spontaneously.

Indeed they do, William. I replace them all the time with no valid reason
found for their untimely demise, and no bounceback a week later.

Thanks. That makes me feel a lot better.

May the humble fisherman make another request of the magic flounder?

My gut feeling is that the ICP was slowly deteriorating, choking off the
current as the unit played, then "recovering" as it sat overnight.

Have you seen anything like that?