The worm gear that is inside the unit may need to lubricated,, remove the bottom case and lubricate gears and shaft with a teflon or silicone based lube. also on my sailboat when this happened I found that I needed to re-calibrate the unit - take the boat out into a very calm and open water and ' spin the boat ' in CAL mode ( this will re-orientate the internal compass ).
have fun on the water and have a great night,

c...