It is a known problem with the circuit board that has the switch on it. My 332C did the same thing. It got to the point where I had to pull the fuse when it was not in use. I upgraded to a 27C and the problem has gone away. The only issue with the 27C is the display locking up in shallow, weedy water. The latest beta software release finally addressed the issue two years after I first reported it to Lowrance. I reported the problem to them just after the unit came to market.

Lowrance will send you a newer unit but it will probably cost you.