I agree that it sounds like a xducer problem.First where is the exducer located?
Ideally it should be in front of the keel so that you see the shoaling before the keel finds it.My xducer was originally installed next to the raw water intake behind the keel okay if you are at sea or sail in deep water but not good for shallow water sailing.
I think your depthmeter goes erratic when the keel starts hitting bottom and stirs up the mud,sand ,etc.
I moved my xducer forward of the keel and shoot thru the hull with a Humminbird fishfinder that shows a pic ot the bottom as well as depth and as a bonus fish.
Dianne and Chuck Burke S/V NiftyNickers C37#139