I think AJ gave you good advice. Just last week I noticed that the volume pedal I use with my MS60 behaved pretty much the same way as you described. I took the Yamaha pedal apart so I could get at the potentiometer and then liberally sprayed a commercial electronic contact cleaner into the small openings on the pot, rotatating the control shaft at the same time. I let things dry off and then reasssmbled it and it's been fine ever since. Hope this works for you as well.
Paul C.