Your check shows it is not the resistor - which is unlikely since a resistor is inherently stable - it would either have nearly the correct resistance or fail altoghether (open circuit). Hard to imagine it changing itself to the wrong value.

I had loose or corroded connections at the motor which made it run slow and/or uneven. Also the bearings were dry. I cleaned up everything and soaked some oil into motor bearings - twice - with the result I got about 3 more years out of the fan motor.

Eventually I got tired of dealing with it and sprung for the new motor which of course works reliably all the time.

The original motor took a turn for the worse after a very heavy rain which filled up the air intake faster than the drains could empty it.