Some things that might help. Keep your fan aon high speed at all times and use your thermostat control to control air temp. Leave the thermostat on as high a temp as possible. This will allow the A/C to cycle on and off. Any ice forming during the on cycle will have a chance to thaw during the off cycle. Evaporator icing was a problem for all manufacturers during the 80's. None of the solutions were very successful. The higher the humidity the worse the problem