Two mixers both with an Audio loss problem at same venue

Hi, I work at a venue with a 750DJM and an 800 DJM.
The 750 for some time has randomly suffered audio drops and then spikes back up again. This happens across all channels at random. it mostly impacts the master output volume but you can see the individual channel drop maybe 2 lights at times with this. Spiking the master gain manually often brings the sound back, but not always. This has happened with various dj's using different equipment, some on USB, some on Rekordbox, some on Sorato.

We changed this out for an 800 DJM thinking it was the mixer. Within 2 hours of using the 800 DJM the master out dropped to just 3 green lights with the master volume at maximum.
The individual channels were still showing full volume.
even with turning the mixer on and off again this changed nothing.

please can you help!! Is this the venue's sound system damaging the mixers? or is there something not set up right?