I have an old 97' Town car that will not heat to operating temperature. It does blow warm air out of the ducts and defrosts at least moderately, however the temp gauge will fluctuate based on how hard the engine is working it seems. When i come to a stop it starts to cool down quite rapidly. This would normally point to a bad thermostat. I have replaced the thermostat twice already, and it hasn't made a difference. Is there any other way the engine could be losing heat so fast? I don't know what to try next.