1995 Ford Windstar heater core or blend door

Heater blows hot while driving but blows cold when at idle, temp guage nears overheating then drops down to normal operating temp on its own. Fan seems to be working properly. Thermostat changed, upper and lower radiator hoses changed, heater core flushed, water pump changed. Coolant level in radiator full, overflows when cap removed and completely cool, cap was changed. Thinking maybe a partial blockage in heater core or blend door not working properly.