I have a Humminbird 597CI HD which will not read the bottom on the unit, it jumps all over the place, even when the boat is stationary. I have tested the unit on two different transducers and it does this on both. I have uploaded the most current version, same problem. The unit was purchased new and only has 36 hours on it. I have seen other folks complain about this doing a google search but unable to find the cause. I have bench tested the unit and placed the transducer in a deep 55 gallon drum, same problem, everything else seems to work fine. Any suggestion? Thanks