Noise Problem. Both Comms Breaking Squelch

Another long shot is might the master switch or master relay be
intermittent? Normally I'd think any inductive kickbacks from motors
etc would be ballasted by the battery - as long as it stays connected
to the main bus. But if there is anything intermittent in the master
contactor system, any inductive load will kick into whatever
electronics are down-circuit from the intermittent.

A classic example of this is if a master contactor should stutter
(like from a weak battery) when the starter is engaged, the collapsing
field from the starter could generate enormous voltage spikes which
could wipe out any electronics that are also turned on. This is
probably why we are told to turn off radios etc before starting.