Looking for advice troubleshooting Humminbird 898c HD SI fish finder. GPS is working fine with lakemaster card. Issue is with sonar. On down imaging, can read the bottom occasionally at slow (less than 4mph) but digital readout of depth will start to blink. When that happens the DI graph stops moving and will restart moving when depth is found. At times the depth reading will stop blinking and have a crazy value (say 250 feet) but the graph is showing correct depth. Issue repeats on side imaging and 2D sonar. I have done a rest to factory defaults as well as hooked the unit up to a separate battery to isolate. Diagnostics on the unit report everything is fine. Have also seen the issue with the motor off so don't believe interference from engine. Thought the issue would be not having unit mounted low enough in water but with the issue happening at very low speeds that seems questionable.

 

Unit is mounted on back of bass boat. Thoughts on additional troubleshooting steps. Considering a Lowrance Elite-9 Ti2 as replacement. Thoughts?

Sorry posted in wrong section.  I re-posted in marine electronics but could not determine how to delete the original post.