2004 Honda civic heater and engine overheating.

I have a 2004 Honda civic. My heater only works when I
drive about 40 mph or more, but not always. Sometimes
even while moving it will blow non heated air. It blows cold
air at stop lights or when going slow. When cold air is blowing temperature gage show engine overheating. All of a sudden hot air could start blowing and then temperature gage show engine temperature goes to normal.

I replaced the thermostat and it seemed to work
Okay for a month, then started this nonsense again.
Anything else I can try?

Thanks.