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Normally this is caused by dirt and debris built up between the fins of the coil on the underside. It causes the water to stop there and drip down instead of making it's way to the pan. You would need a wire brush or fin comb to get in there and really clean it out. You have to be careful not to bend these fins though or it will cause the airflow to be restricted and you will have even more problems.
Another thing that could cause this is if there isn't a PVC trap installed on the drain line just outside the furnace cabinet. It is a horse shoe looking thing made out of 3/4" pvc. If this wasn't installed on the air handler drain it would be leaking all the time and is odd if this is the cause that it just started doing it 5 years later.
The unit has a pvc trap on the condensate drain line just outside the air handler cabinet but when I remove an access cap I see very little liquid moving.
I will try to clean and rinse the fins of the coil again. I used a 3" paint brush previously to do both the top and underside and there was a lot of dirt removed, but apparently it did not cause enough dirt to be removed between the fins. Where would you suggest I can find a fin comb ?
http://www.bestbuy.com/site/Frigidaire+-+Fin+Combs+for+Most+Air+Conditioners/9789352.p?id=1218175317739&skuId=9789352Home depot would have something like this as well.