I have a Hummingbird 400 series depth/fish locator. I can get the water tempt., and bottom view. The problem is the depth reading varies from 3 to 30 feet to 1 foot at the same location.The reading is mostly 1 foot even in deep water.The screen often goes black or has black verticle lines in it every so often, The unit is about 4 years old.Any ideas?? 
