Is this sander belt driven? If it is, and it happens with the replacement, take the belt off and see if the motor starts and runs properly in the cold. If it does, turn the pully on the tool by hand to see if there is resistance. If there is, it could be (as previously noted) the lube in the bearings (or sleeves).
I don't think I've ever seen a new motor bind up because of cold.
A cover, like a towel, with a shielded light bulb under it will keep a tool like that warm.