Suggestions for starters:
1) Make sure your unit is running the most up to date software. Check Humminbird's Web site.
2) Load test your battery. Just because it charges up doesn't mean it's any good.
3) Make sure all connections (electrical and transducer/accessory) are good and tight.
4) If your sonar is not hooked to it's own dedicated battery, then get one. It doesn't have to be a big battery.
5) If your sonar wires run parallel closely to any other electrical wires, then cross them at right angles (like a cross).
6) Make sure when your boat is in the water at operating weight, that your transducer is adjusted so it's looking straight down.
7)Try bending the power cord connections outward a little to get a better connection. WARNING:Bend one at a time or you will blow a fuse. Don't ask me how I know this.
Lastly, if you've done all of the above and are still having problems, call Humminbird customer service at 1-800-633-1468. Be polite and calmly explain the problem to them and ask them what they suggest you do next. They'll get to the bottom of it. 9 out of 10 times, it's the battery, connections, or electrical interference.
Good Luck.