Ok I figured out exactly when this happens.
It happens when I am using the Dot as a Bluetooth receiver, and playing STREAMING audio through my iPod Touch 6 to the Echo, and from there over the Aux port to my Soundlink Mini 2.
After two minutes the audio cuts out.
If I'm just playing mp3 files, the audio doesn't cut.
The only way to make it go again is to either
disconnect reconnect Bluetooth
or
unplug replug the audio cable