Have HB 587 DI. When idling along the screen goes crazy when using DI or regular sonar!! the picture read out will go from 30' to 480' (reading in lower right corner) so can not see any structure. Also the water depth will very all over the place from accurate reading of 10' to anywhere upto 160'. Also while setting on trailer the speed will conutious switching anywhere from .0 to .4 miles per hour. Help please.