Condensate Pouring From Spiral Duct

Had a new customer call for regular maintenance on 3 RTU's. Goodman 10 ton gas packs. 5 years old. Stated one leaks water from supply duct since date of install. Only when in cooling mode. Looks like the installers used a curb adaptor on top of a an existing curb adaptor. The new curb adaptor is 180 degrees to the existing one. These are the cheapest made RTU's I have ever seen. Cleaned the coils, changed filters and belt. Tested the drain pan with 2 gallons of water. No leaks. Unit insulation was damp on supply and return air compartments. Light rust on blower motor, pulley... After running for a hour condensate is pouring from the spiral duct like a stream. The condensate pan is sweating like crazy. It also flowing out the drain line the same. The two scroll compressors are sweating too. It looks like the condensate is being suck from the compressor compartment back into the return air section. Not sure if it has a high static blower. Blower discharge dumps into a 14" space on one end of the curb and has to travel 5' to the supply drop. Static pressure at the drop and at the spiral ranges from .2 to .35". Which is way too high for the standard blower motor. I really think its going negative somewhere in the curb. Even the bottom pan of the return compartment, which is hanging over the curb adaptor is sweating. Dripping onto the curb. The installing company sent a ton of techs back and couldn't resolve the situation. I noticed they tried to caulk the seam separating the compressor compartment to the SA and RA sections. They apparently walked away saying it was due to high humidity levels in the dining room. Ironically the second unit that serves the same area is 20' away and doesn't have a problem. Same duct design. Though that and the 3rd unit are mated end to end and side to side with the existing curbs. Thus my assumption for a negative condition in some area of the curb adaptor. I'm only a tech, not an engineer. I pretty much told the customer he would need to have to resolve it with the original company that installed it or contact Goodman direct. Like I said before the design is crap. All the cover screw hole were pre-stripped from the factories. I did go in with larger screws to lock them in tight. The metal gauge is paper thin. It still bugs me that I don't have a definitive explanation for the condition. I won't be going back because my company has pretty much washed their hands in getting involved with it. I believe I have assessed everything correctly but have hit an area where I don't have the knowledge to proceed further. It sucks going into grey areas that I can put my finger on and have a 100% correct diagnosis without facts to back it up.