Heater checks out ohm wise gut looks dark in the glass.Rapid changes in evaporator fan motor is not normal .This means mother board is bad causing this and the board puts it in defrost.
You mean thermistor was 3.8 ohms at room temp. And looks like the picture in sent you?3.8 at room temp would be about right like on the chart i sent you.
Normally with heater or not defrosting you would check out heater from the board itself while it is install.By using the attachment I just sent you.Also this checks out the heater and safety thermostat .That is in series with the heater .iIf the safety is open you will have a bad heater or thermostat with checking off the board.