Welcome to the forum.
You're not alone with the problem - happens with the D40s too. On mine, a resistor got too hot ... burned out and it was all over except for fan speed 4. I bought a replacement off eBay (ages ago now) and it's been right as rain ever since, and it's always running when we're in the car.
I have wondered if additional drag on the fan may cause more current to flow through those resistors which will undoubtedly heat them up. If there's debris in the fan area, or perhaps even just a combination of moisture and dust, you'd think the motor would have to work harder to maintain fan speed.
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