I had a similar problem with my LX-3 this year. It would show multiple, unstable, weak returns throughout the water column and it did not show a clearly defined bottom return. The battery had just been charged and the voltage checked fine. The power cable had continuity and the transducer was clicking nicely. IR on/off had no effect and gain adjustments just made things even more confusing.
I got it to work perfectly by simply squeezing the power connectors at the battery terminals. Apparently, they did not have adequate contact. The connections need to be pretty tight, not loose.
Give that a try.