one problem, many questions.
1. is the unit above the lid? if so is it level? have you verified it's not condensate from the evap. pan?
2. what is the air velocity thru the unit? is it too high and drawing the moisture off the coil? have you done a delta P across the fan to get CFM yet?
what is your air side delta T?
3. is the duct in question properly insulated? if hot air contacts the duct moisture 'may' form. the first process of cooling is dehumidification= that moisture will go somewhere.
Just some random thoughts...I'm not an expert,but I did stay at a holiday in last night.