That's one strange thermostat.
Dave's Place - Dometic Manual Refrigerator Controls
Between the thermostat and the 120v heating element is only the thermostat points. Since you are on your third thermostat, either you are buying substandard thermostats or your 120v heating element is pulling too many amps, ie. the heating element may be somewhat shorted internally.
If you have access to a clamp-on amp meter, you might want to find out what the amp draw to the heating element is running as it is doubtful that three thermostats were bad in a row. If the amps are higher than normal, time for a new heating element.