House is 21 years old and never had a mold problem till we replace the a/c unit 3 years ago. Do some units produce too much moisture? The new unit is a well known brand. Look in some of the ducts today and can see mildrew. Is there a solution?

No, A/C units do not produce moisture, they remove moisture. So it may be that you got a unit that does not remove enough moisture. This is not a simple troubleshoot for a diy'er.

If I were you I would call local reputable companies out for their opinions as to what's wrong and what it would cost to fix it. These sorts of estimates are free.

Over sized units will cool ,but will not move air across coil slow enough to remove moisture ,have unit sized as previous note said, but moving blower down from high speed to medium high may help .