It sounds like you capacitor is going. Most blower motors are capacitor start. Alot of times the CAP is located on the motor (It looks like a lump on the side). the others the cap is usually located by the power control board.

the first image attached shows an on-motor cap. You will have to remove the metal case to change or check the CAP. Does your multimeter have a capacitance test option? That would help.

Also, you can check amp pull on the line going into the motor.

-10c can affect motor performance.

 

If the capacitor is that cold, is may not be operating fast enough at a high enough voltage.