I would first start by having the antifreeze tested for temperature rate, just incase you have weak coolant. Then I would replace the thermostat. Sometimes thermostats are summer ones, and start seizing when cold temps hit. If none of these do the trick, I'd check the heater core. It may be plugged. You can do this 2 ways. 1, you can disconnect the 2 hoses from the heater core at the firewall, and flush the core. The water should flow through easily, no blockage at all. 2 Get the whole cooling system flushed and pressure tested. It should hold 14 psi easily. . I have a few questions though. Is your fan running continuously? Is your coolant level going down at all? I'd say it is a heater core issue though as I have an 02 saturn that used to do the same thing and a new heater core fixed it up fine. Hope this was helpful