If you have tried two new units and they both acted up in the same way then that tells us something. What do they both have in common on your boat?
Are they both hooked up to the same battery? If so then what other equipment is running off that battery? Could your alternator on the motor be putting out too much voltage or amps? That may cause the unit to shut off or to blow a in line fuse. You did install a fuse in-line to protect the depth finder.
Another thing they probably have in common in is the TRANSDUCER. Where is the transducer located or mounted on your boat? It maybe that you are getting air bubbles between the transducer's face and the water. You might have someone drive your boat and increase speed while you have a look at the transducer while the boat is moving. You may have to hang your head off the back end of the boat to see how the water flows by the transducer.
If you have an aluminum boat with rivets they will produce air bubbles and you may find that the transducer is in some turbulent water. You may look at the back of the boat while it's underway and find some more laminar water flows and mount the transducer in that area of your transom.
Some transducer need to be mounted a little bit deeper in the water to ensure good contact with the water at all boat speeds and plane angles.
Read the manual and study the parts that talk about the transducer and how it works and where it's mounted.
Those are just two ideas that come to my mind.
There is a slim possibility that both the units were defective in the same way. That could have been the reason but I would bet on the transducer mounting position or some electrical interference coming from something else running on your boat.