@heinzelrumpel: when your streaming just stops, it seems that the bluetooth signal is lost. Do you have your Pi in a solid case? Do you have an external power supply for the Pi or only the power supply from the AMP that is also powering the PI. The reason for breaking bluetooth connection might be because of lack of power. Try to additionally connect a power supply directly to your Pi for testing OR if you can, use a powered USB-Hub and an external bluetooth dongle that works much better than the inbuild bluetooth on the RPi 3.
@mnvelocitypilot & tonyw: great to hear it is working now!