It definitely sounds like you are getting a partial frost closure on the vent.
Quite often this will occur in homes that have more than on bathroom, where the problem bathroom is a distance from the main vent and has an auxillary vent.
Main vents are required to run undiminished in size from the main drain up and out through the roof. Auxillary vents may be reduced to 1/2 the diameter of the drain they serve once it is 6" above the flood level rim of the highest fixture served, but must not be reduced below 1 1/2"(min 2" if serving a water closet(toilet).
In geographic climates subject to freezing the auxillary vent is required to be enlarged to 3" at least one foot inside the structure and run 3" up through the roof to prevent frost closure.
In some newer homes that have heavy insulation and well ventilated attic spaces the incrase to 3" should be made as soon as the auxillary vent enters that attic space.