Fall came around and it got cool so the humidity problem went away last year. Fast forward to this summer and of course, customer called again. Figured I'd update what I found and what fixed it. The stacks,or lack of, going up into low sidewall registers were all leaking, mastic sealed those, put an attic tent on pull down staircase and raised air handler up to get a better fall on drain. What I believe was happening was the dominant supply leakage was increasing infiltration through the attic hatch and that hot moist air was cooling down to close to dew point temp and raising rh. Also, lowering the fan speed made the air handler/ducts colder increasing the sweating. Once I fixed the other stuff I raised the blower speed back to 400cfm/ton. I think this problem had been ongoing with the old system as well as the new one, but last year the dp, rh, and temps were all making the conditions just right to make all of the problems more noticeable. It was a great learning experience for me. Thanks to all that replied.
Edit: when we installed the unit we also sealed all the return with mastic and not the supplies which made supply leakage even more dominant.
We have the same problem with one of our customers. Got the unit 2 years ago and she never has been real happy with the humidity. We tried everything last year, tomorrow we are downsizing her unit from 3 1/2 tons to 3 tons at our expense. This should fix the problem otherwise she needs better insulation. Its not something we want to do but in a small town sometimes you have to bite the bullet to save from word of mouth.