I have a Humminbird 998 CSI unit on my console. While we were on Lake Guntersville last week, the depth finder didn't work for pretty much the entire trip. It was stuck on 3.4 ft no matter where we were. I called Humminbird tech support and they said to take the maximum depth off of automatic and set it for 10 feet more than the maximum depth. So we set it for 40 feet. It didn't help. Got back home and went to a local lake yesterday and it worked fine. Any suggestions?