GE Monogram SBS-Refrigerator- Freezer Coils Icing up

My side by side refrigerator- freezer tempature is always changing, some times up to 50. when it should be at 37. The Ice Melts in the freezer! and the coils inside the rear of the refrigerator Ices up. We’v hade the main board replaced and it still craps up. Please advise.