Most boat manufacturers do not provide heavy enough wiring for today's power hungry electronics ... Regardless of what battery is connected - if the wiring to the Sonar unit is not big enough, the wire will not carry the consistent amps needed ...
Your 2d and depth issues sounds like the existing shoot thru xducer is either coming loose from its epoxy mounting or just plane wore out (transducers elements vibrate - vibration is physical energy - physical energy wears stuff out) ...
Disconnect the shoot thru and Y cable and test the 2d performance from the stock SOLIX xducer ... If things are better you might think about replacing the shoot thru xducer ...
Rickie