1989 Nissan Maxima Heating System

I have a 1989 Nissan Maxima with 162000 miles on it and was just wondering what is wrong with my heating system. When you turn on the heater its gets warm,not real warm as usual heaters do, just like a lukewarm. When yo go over 45 the heater is warm and when you slow down to 45 or lower it gets ice cold and freezes you out. I am in MN and need heat, what could this be.I'm thinking either heater core or thermostat, hopefully thermostat because heater cores are so expensive. Thanks Daniel