This one's tough. It sounds like there might be some water in the switch, but since the Filco has plate-mounted switches, they are not easy to take apart. What you might want to try is taking off a cap and taping down the stem about halfway to allow the switch to dry out (if there is water in it). Otherwise, you will have to desolder the switch and take it apart to get better access to it. Filcos are tough to open up, as well, but there is a guide somewhere here; maybe someone could chime in on where it is.