Pay attention if depth readings shift during times when moving from spot to spot at higher speeds. If this is the case then you have air bubbles disrupting your sonar signal causing inaccurate readings or a loss of bottom lock for the unit. If over this weeds in shallow water, this at times creates havoc with sonar readings because of the wide variance of signal returns.
check the voltage at the head unit.You should have a minimum of 11.7 volts. 12.5 to 13.5 is optimal
lower power input can affect the overall performance of your unit.