Before replacing: Remove the switch and find the small hole in the housing. Spray some contact cleaner in it and work the switch numerous times. That usually fixes it. I also follow up with a shot of Tef-Lube spray to keep moisture out.
+1

Had the same issue with my clutch switch recently, and a quick remove and clean was all she needed. I used a small piece of 1000 grit to burnish the contacts a little through the hole, worked great.