Seems like the water from the evaporator when the unit goes on defrost, doesn't completely drain before the unit comes back on. The drain tube isn't stopped up. I think the pan is not slanted enough on each end and what water doesn't drain out, freezes and eventually stops the drain hole up. Is there a way to raise the drain pan up slightly on each end to prevent this?
Answer Jim, Removing and reshaping the defrost drip pan probably isn't going to solve the problem. But there are options and alternatives, you could install 819043 Heat Probe kit, to keep the drain hole "open" in the drain pan, and or install a "duck bill" 22001900 in the end of the drain tube in the compressor compartment to keep the hot air from the compressor area from migrating up the tube and causing ice to form in the drain hole.