Hello, in case anyone is still following this, or stumbles across it, I have an update of sorts. I've been around, and around, and around, and around... with Amazon tech support and customer support. What I am told is this is a "known issue" affecting "some" customers. Apparently, it has nothing to do with firmware, etc. One theory from the tech support folks was that it had to do with internet drop outs, but that was disproven. The latest I've been told, two weeks ago, was that a fix has been written, but it will take "a few months" to reach all customers.
Interestingly, since Spotify came onboard for multi-room, the problem occurs less frequently. Rarely on Spotify, fairly often on Pandora and Amazon music.