I used to fret quite a bit over the humidity - and it was all over the place as you describe. one day it just magically held rock steady. I believe it was two likely reasons - me constantly checking the unit - and the humidityeventually stabilizing once I filled it with cigars. It seems to eventually become a "system" if you just let it be, and humidity seems to hold well for about 6 weeks before I need to add more water.

Bottom line - less is more...

BTW - welcome!