The problem you describe does sound like improper air flow through the system. Sweating can be caused by slow air flow accross the evaporator and supply. Condensation can be seen on the registers as well.
The air flow needs to be measured at each supply register and accross the evaporator. Have you measured the air temperature accross the evaporator? A low temperature coming out of the evaporator will cause condensation in the supply and is a symptom of air flow restriction or an improperly charged system.
Does the system tend to short cycle? That could indicate an oversized unit for the home and you you will have problems with removing humidity.
Is the condensation drain clear. The attic installs can be a pita. and an indoor temp. of 68 degrees in south florida can make humidity removal a probem for even the most balanced systems.

Good luck and keep us posted.