I have this model Maytag Plus and every late spring when the weather outside starts getting humid the inside of the fridge side starts sweating then all the coils freeze and I the unit stops working until I have to turn the unit off and manually defrost the unit and restart it. I have replaced the defrost timer and it made no difference. Everything works fine all winter long. This problem only happens in the late spring and throughout the summer when I am not running my house ac. Any ideas?

Sounds like you have an air leak which is allowing excessive amounts of moisture-laden air inside the refrigerator. Then what would happen is the evaporator would frost over more rapidly than the defrost system could keep up with. Possible sources of an air leak are doors not closing/sealing due to shelving or food obstructions, torn door gaskets (usually along the bottom of the doors), or maybe the defrost drain tube has come apart back by the compressor and the "trap" is no longer there.