Frigidaire refrigerator - Replaced the thermostat, thermistor, and main board without any success. Heater element was either coming on too often or staying on too long and always accomplanied by error code d/F. Lately, disconnected the hearter element and frig runs fine for about 30+ hours then frosts over. Allow tiime to defrost (about 2 hours) and then everything is fine for another 30 or so hours. What else could be causing this problem?