Your problem is that your gas motor is drawing current away from the sonar unit when you crank the gas motor because you probably have your sonar hooked to the same battery as your gas motor.
Also,
You are getting interference from the gas motor.
Get yourself a dedicated battery for the sonar and only run the sonar off of it.
It does not have to be a big battery.
Also, don't run the sonar wires close to the gas motor wires.
That should solve your problems.