If the fan motor bearings get a bit dry, it slows the fan down. The resistor pack is cooled by the airflow. When the fan runs too slowly, the thermal fuse blows which shuts it off. Full speed is not affected as the resistor pack is out of circuit. I think the main problem with these is age and the fact that the fan runs really slowly even when the switch is off. The slow speed only stops when the air control lever is fully down.

Where did you get the thermal fuse from? I replaced mine about a year ago but it's blown again.