That little power control ring is out of plastic with a metal contactor underneath.
Happened on one of mine F3´s and I simply removed the cover, and mechanics and cleaned the ring, glued it carefully together with cyanoacrylate and made sure it did not built up additional thickness or affected anything else.
I did put a tiny amount of silicone lubricant under it and put everything back and it's working like charm.
Very smoth nowadays.