My switch was doing the same thing. What I did is remove the switch, lightly sand the contacts as Paul suggested, clean with contact cleaner and then apply Di-Electric grease to the reostat. The Di-Electric grease lubes the reostat so it turns easily but also (sez so right here on the can) "protects electrical contacts and prevents rust and corrosion". My switch now works better than new.
Actually I use this stuff on all of my electrical connections, spark plug wires (make them easy to remove, tail lights, headlights, etc). Seals out moisture so the contacts to corrode or short out.
Johnson Rod
Southern California