Fading in and out

Neither? These are digital speakers, they're not supposed to fade at all, unless someone is playing with the volume, And in any case, I don't think you're overloading the system, and I also don't think it's your internet connection. But what I would do is, within 10 minutes of this event happening again, submit a system diagnostic, and post the number here, for the folks at Sonos to look at.

I have found this happens when music is being controlled in any of the rooms via an amazon Alexa/Echo. Sonos pairs back the volume in all rooms whilst Alexa is being given any type of instruction. Apparently a fix is to remove Alexa's knowledge of the rooms/zones that you won't want to control via Alexa so it won't dip the volume.

Ah, you're talking about the "ducking" feature, which is what the system does when Alexa is being spoken to. There's quite a few threads about this, and Sonos has acknowledged that they're aware of the concerns. I'd encourage you to read those threads. Just search for "ducking".

I don't use Alexa and I am having the same issues. We moved homes recently and when I set the system back up the problem started. My Play 3 is always playing music, but the wired speakers will fade in and out. I just submitted diagnostics: 8227881