The problem generally occurs because excessive debris has made its way between the key switches contact points. Replacing the key switch will work but may not be the most convenient fix as it requires soldering/desoldering. You could try washing your board, see the following posts for some ideas:
http://deckkeyboards.com/forum/viewtopi ... hilit=washhttp://deckkeyboards.com/forum/viewtopi ... hilit=washIMPORTANT: You really really want to make sure it is completely dry before plugging it back in!