heater is blowing out cold air

My 2004 Mercury Sable heater is blowing out cold air after a couple of seconds of blowing out warm or hot air. The thermostat warms up and does not over heat. I also do not see a leak spot when I am coming or going. What could the problem be?

-No Leak spots underneath the car
-If on the highway the heat will come on, when in town stopping a lot causes it to cool

Thanks in Advance