Your system is low on freon charge. The same will happen in a standard air conditioning system as it gets low on charge. What happens is when the freon reaches the evaporator coil, it flashes over from liquid to vapor state, and the evaporator gets too cold, and the water in the air freezes to it, and eventualy frosts completely up. Chances are you have a slow leak in the system someplace.
UNLESS.. your air flow has been restricted somehow. Check for a very dirty furnace filter, slipping fan belt, etc.Good Luck!