First, if you're in this thread because your AirPort Express dropped with a yellow light, my post is not for you.
If you're in this thread because you've had a random drop of AEX connected devices during AirPlay playback from iTunes, read on.
My network:
Airport Extreme 5G with ethernet connected Airport Extress (old), and wireless AEX (old)
going to
AirPort Extreme 5G to extend the network to AEX (old) wired, AppleTV (new) wired, and AppleTV (old) wireless.
iTune playbck would drop ramdomly to all AEX devices after a few minutes of play. All AEXs green.
I gave static IPs to all the AEXs and the Extreme that extended the network. It didn't corrent the problem.
I set IPV6 to Link Local, it didn't correct the problem.
I pinged all the AEXs, it didn't correct the problem.
I got a prompt from my AppleTV (new) to set my credit card. I did. Everything has worked since.
All good now for several days. Go figure.