Tonight I asked my Echo (full sized model) to play a local radio station. It started playing for a few seconds and then went silent for several seconds. It continued to play for a few more seconds followed by several more seconds of silence. It did this about 4 or 5 times and then went completely silent for about a minute. Then I saw that the blue light ring was spinning and I heard the normal startup tone and it said "Hello" like it just booted up.
I just wanted to report this strange behavior. For whatever reason, the Echo decided to reboot itself. After that, the radio station played just fine.
Maybe it's a new thing that Amazon is doing, having the Echo check itself for problems and then rebooting itself. Pretty cool if that's what they intended.