Heating system problem

i own a 99 Ford Mustang V6 3.8L. recently the heat in my car stopped working properly. first the middle two vents on my dashboard quit blowing air. and now the heat seems to cut in and out. it is mostly cold air that comes out. the check engine light isn't on and my car doesn't seem to overheat at all. all my readings on the display are normal. the blower motors on the vents closest to the doors work perfectly. they just don't always blow heat out. what could be the cause for this problem? Please help.

~A freezing girl in wisconsin

The heat cutting on and off is often a symptom of low coolant level. Check the overflow tank and the radiator itself first.