Freezer evaporator keep frosting
Partial frosting is a common sign of loss of refrigerant pressure, i.e. a leak in your evaporative cooling system. You've already addressed the other likely causes of frosting (great!) but that leaves you with the likelihood that it's a leak (doh!).

Refrigerant chemicals are environmentally toxic and potentially hazardous, so at this point you should either have it professionally serviced or replace it (disposing of the old unit properly).