I have recently noticed that there are many posts on this board related to vent pipes becomming clogged or frozen. When the toilet spits back, that suggests to me that the air is not finding its way out of the stack. Since it cannot get out of the way of the water, it bubbles up to the closest escape point. You might want to just give this a little time and see if the next thaw solves the problem. Other than that, I would focus on the soil stack ventilation.