refrigerator and freezer warms up

The fresh food section freezes the food,then both the freezer and refrigerator will warm to where the ice is melting in the ice cube bin. When I remove the back panel inside the freezer coils are extremely iced up.I unplug the refer and let everything thaw out and start back up and every thing gets back to normal, it takes a couple of weeks and it starts the same process again…is it the defrost heater? Appreciate any help, thanks, Mike