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Montana Sky.
My money is on too much static air pressure; the small ductwork has too much restriction for the outlet airflow. This causes the air to slow from the unit, the temperatures drop on the face of the evaporator coil, causing a freeze up. All of the Montana's are marginal, some freeze and some do not, from what I read. The fact that it works fine when the dump damper is open and you run it on high, reinforces my opinion. If you could 'cut in' two or maybe three more outlets, it should work better, using the ducting. I would bet that your T.D. is high, probably around 30 degrees or better with the dump damper closed. That is too high, it shows insufficient airflow. You want around 20 to 25 max. degrees. (T.D.= temperature difference between the outlet air from the evaporator coil and return air, or space temperature.)
It's all airflow.
Good luck.
Ozz
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