Sounds like trolling motor interference to me.  Now call Humminbird’s Customer Resource folks at 1-800-633-1468 and talk to them about what to do.  Generally they will need to know what Humminbird equipment you have on your boat and may have you do some re-wiring or change a menu setting or two.  If these don’t help enough they will probably send you a trolling motor interference kit to install on your boat.