Go to Altronics, and get some contact cleaner ( or you can use RP7/CRC ) . Open the switch body and give the switch a good flush out, amazing the crud that will come out. give a flush out with ethanol, then apply a good coating of Silicon grease (all available at Altronics I think) put switch back together and away you go.
Reminds me, I must do my switch as well.
P.S. separate the switch halves carefully, there are locating pins, but no bits that will fall out.