Sounds like possibly the motor has a thermal protection malfunction.Motor n capacitor seem good, I can watch the switch rotate to the on position but as the blower run it rotates back nowhere near the off position n it shuts off????? Don’t understand why
Its preset by the switch.What is a thermocouple reading on a wood furnace? The fan capacitor is a start capacitor not a run capitol, if it starts it's good. Your fan motor is bad, it is shutting off early on internal thermal overload.