I don't know where you're at. Where I am there is a lot of dust in the air and when there is a lot of humidity, the dust collects (and builds) between the rotating motor part with the fan blades on it, and the fixed motor frame.
I usually DISCONNECT POWER, and use a can of spray contact cleaner to blow the dust/grime out of the space
between the two. A lot of times the grime acts as a drag on the rotating part.
You may have a motor problem, or a circuit problem, but I'd try my fix first.