If the temperature is not stable your heater is too small, your lung room is too unstable.
One option would be insulating of your box, but could screw you up when the lights turn on ;-).
Another option (if your lung room is dry) would be more inlet.
Nonetheless, I would try it without the heater and compare. As you can see in your measurement, your RH and temperature are directly linked. So your humidity is most likely caused by temperature.
It seems like your heater needs to long to heat up your lung room. So put it in there and turn it on before the lights turn out.. or something else. If your temperature does not drop too low without heater and you got no mold problem ; I would turn it off.
Just as reminder the positioning of your sensor and airflow in your box are also crucial.