side by side defrost issue

My fridge was not cooling and the freezer side was freezing up (I have a Whirlpool Side by Side Model number ed5fhexsq00). We replaced the defrost heater and the defrost thermostat. It worked for about ten days and now it has the same issue. It does not have a defrost timer but it does have a motherboard. Can you give us some guidance on what to do now?