heater on 2002 Dodge Caravan

Heater was blowing hot air and then cold air. Checked water level, it was low so added water. After running for 15 minutes in garage heater seemed to work fine. Went on an hour drive and heater started blowing only luke warm air then only cold air.

3 answers
after having the symptoms and changing the thermostat without any luck, I took the van to the shop. The tubes that carry the coolant to the back blowers of the van (dual heater controls) were leaking. We had them replaced and everythin is fine now.
you are losing coolant from somewhere. If you do not see any leaks get a coolant system pressure test. my guess is probably head gasket needs to be replaced.
same problem...added antifreeze and water to raise levels and was fine for a while...happened again and I changed the thermostat...still no heat...any answers out there???