last year my eagle 320 quit markig fish , even in good perch water areas. sent it back to cabelas and got my money back. so then i put my eagle 160 back on the boat. now this year it will track bottom only to 30 ft of water and then quit marking the bottom. the sensitivity drops down to 48% and somtimes the depth reading show 200 or 300 ft of water. isolated the fishfinder by putting it on the other side of the boat and hooked it up to it's own 12 v. battery .still the same readings . ??? i hate to fish if i dont see fish.