I have an 898 on the bow with the transducer on the trolling motor. A 798 unit at the consol with the transducer on the stern. It is a 18 foot bass boat. I have noticed that when both units are running in side scan mode I get interference on the display. I appears as lines running accross the display about a half inch apart. When I shut down one unit the lines dissapear on the other. Is this normal? Can anything be done to minimise or eliminate this?