Thanks. I agree the issue could be the QU16, however it’s really only been used about 30 times and therefore is basically still almost brand new. The other thing that makes me think it’s not the mixer is that it only is cutting out when live music is being played. On breaks I play music through my phone thru the ST3 channel and it’s perfect. In my opinion for all speakers to cut out it has to be something that is either originating sound like the mixer or else the xover since the mixer sends all sound thru it. Since I’ve eliminated the xover that leaves the mixer but since we have noticed it’s commonly happening when the bass player kicks on his vocal harmonizer, I have been focusing on that as spiking and causing the mixer to temporarily overload.
While I don't normally like to see someone show up with one of those harmonizer pedals as they usually add noise to the vocal input and are overused, never had one cause the issue your having though.
How are they feeding the reference input to the pedal? Could they be creating an internal feedback loop through the systems? Some of them have a mix function where the reference input can be mixed back into the main output.