Does the engine warm up to running temp?, if it takes a long! time if it does it is probly your thermostat it could be locked open, run the engine from cold and FEEL the top pipe, there should be no water running through it at first untill it warms up.
If it warms up O.k (eliminating termostat) and your coolant level is O.k then it could either be your heater matrix is getting blocked or a weak water pump,
steve, February 2005