That's odd - I've never had issues with AirPlay, but now have a whole house Sonos setup which I use instead. Have you tried AirPlay 2?

I don't think so; I set up all my sound stuff before AirPlay 2 was out. I have an Airport base station with two different receivers (one Sony, one Denon), an Airport Express, and a Raspberry Pi running Runeaudio, each of which is connected to its own pair of speakers (or, in the case of one of the receivers, a surround setup). When I stream (from either iTunes on my MacBook or my iPhone) to any one of those, the music will have pretty frequent interruptions; it really shouldn't, either, because my MacBook is typically about five meters away from the Airport Express in the same room. The problem is significantly lessened when I play MP3 music instead of ALAC, and significantly worsened when I play 24-bit audio files, which suggests that bandwidth may be the problem.

Yes - sounds like it. The express doesn't do AC on 5Ghz does it, only N?

Could be congestion - the makeup of your house, the wireless chip in the express going bad.