More than likely it's your defrost system. Consisting of adaptive defrost control (timer) defrost heater and defrost thermostat. Below the evaporator fan is the evaporator coils. Remove the back cover in the freezer to observe the frost pattern.
If it's iced up check the heater and defrost thermostat. If they are ok it's your control board. Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com
Refrigerator not cooling freezer ok 
