We've got an improperly draining A coil in an American Standard downflow fan coil unit.

Maintenance guy allowed to filter to clog, though some initial dust could still be slightly clogging the coil.

It's been cleaned twice with green Diversitech concentrate light foaming cleaner from Michel.

Condensate leak stopped after second cleaning and then started leaking again after a month.

R-22 pressures are correct. PVC condensate drain is clear.

Have others found a better or more effective coil cleaner worth trying?

Fan is variable speed (non adjustable) and air flow does not seem to be an issue.

Do others have experience other than coil cleaning, or items mentioned above, which could cause this 8 yr old system to develop a condensate leak?