Nasa target 2 depth sounder,
I've seen this several times on the NASA kit, it's a simple case of you gets what's you pay for and although we don't see a lot of NASA kit fail, it's cheap for a reason...
Best solution: buy Raymarine ST40 depth. I have a good refurbished system in stock. Or a garmin fish 100 or whatever it's called.,.
How have you mounted a NASA transducer anyway, they don't make a transom mount one that I'm aware of,
Alternative: try relocating transducer wire away from all engine related wiring? It's whole length (hard on a rib I know). Same for display power cables. If this doesn't work perhaps a ferrite or even a low pass filter on the supply cables (I know you said separate battery but it's connected for charging I assume)... In reality if you've reached this far you should revert to option (1)...
If you do persevere the NASA transducer is an unusual 150khz frequency where most manufacturers use 50 or 200.