I'm having a problem with the depth finder on my elite 7 hdi if you go faster than 21 mph it won't tell you a depth and twenty minutes or so after you sit down to start fishing the depth will go from 28 ft of water ( using today as an example) down to 0- 2 ft I don't get it because if you turn the unit off and back on it will read the depth it was before it messed up than twenty minutes or so later it goes haywire again any help will be appreciated thanks