1997 Ford F150 4.6 liter only getting heat occasionally, changed thermostat already problem still exists?

I have a 1997 Ford F-150 4.6 Liter 84,000 miles. The truck takes forever to heat up, when it does I get hot heat, but then the temp gauge starts to drop and the heat goes cold. Sometimes the gauge doesn't drop and the heat goes cold. The gauge never goes beyond about a quarter of the way up. The blower works. I changed the thermostat, what I thought was the most obvious for what seems to be over cooling, but the problem still exists. I checked all lines and fluid is flowing through the heater core as well. At a complete loss??????
1997 Ford F150 4.6 liter only getting heat occasionally, changed thermostat already problem still exists?
Check and fill the Coolant again and into the radiator, because there is a giant air bubble that moves around after you drain and then refill the coolant once. Sometimes on my truck the coolant dows not flow from the plastic overflow reservoir into the radiator, that's why I said to fill the radiator directly again.



I think the factory thermostat rating is about 195 degrees or so, so check the package.
1997 Ford F150 4.6 liter only getting heat occasionally, changed thermostat already problem still exists?
Is the heater core plugged? Or the lines to it?

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