HVAC a Ventahood which has Condensation issues?

In the Oct-Nov time frame, we find a cup of water on the cooktop in the morning every couple of days. It looks to be coming thru the inside of the vent draft and it does not need to rain. The water accumulates in the fan housing. At some point, it spills out on the deck.
The 7 inch smooth duct vent rises 2 ft above the hood, turns at a 45 12 feet to a 45 thru the roof. It is all dry on the outside of the vent. The roof vent is not rusted.
We contacted VentAHood about the $2000 IZTH model. They have "never heard of this" and do not have a suggestion