Your freezer freezes because the evaporator is inside the freezer compartment. Check the temperature, is the freezer compartment getting to -18 or the top freezer shelve working properly?
You may need to replace something called defrost thermostat. The thermostat may be broken and no longer reacting to frozen evaporation by switching on so that the heater can operate during the defrost.
The continuity is easy to test. If the defrost thermostat, normally attached to the frozen evaporator to make continuity happen does not show any sign of live, heater will not come on get a technician to replace the defrost thermostat.