I had a problem (please listen sort of relevant maybe?) where my digital tacho was resetting when fish finder on - eventually I tracked down to 12V supply for tacho came from 12V switched by engine key at remote control box - whereas fish finder power came from battery master switch - once I took power for tacho from fish finder problem went away - I think there was a ground loop issue as slight impedance on ground back to battery via control box.
So anything been changed and since then? Or is problem present from new? As steps suggests check radio aerial connections - ground wire (shield) can break easily at either end or signal wire or could be corroded particularly at base of aerial - check continuity of cable - ie ground of case connected to base of aerial etc with multimeter.
Reproduce problem at home on trailer ie fish finder on, radio on I assume happens whether engine running or not? Or only when engine running?
Then try changing (temp hookup) power to fish finder or radio (whichever not already tried or both) direct from battery and see if problem goes away as interference maybe from other devices or ground . Does a handheld VHF or FM radio pickup interference in boat as well?
Good fishing trip nothing breaks, great trip catch fish.