GE Refrigerator PFSF2MIXBWW defrosts too often and gets too warm

GE Refrigerator PFSF2MIXBWW defrosts too often and gets too warm Need help with my GE PFSF2MIXBWW Profile Bottom Mount Freezer Refrigerator. Purchased ~2010. I replaced the Main Control Board after a hard failure. The unit is cooling again, but over several weeks, the autodefrost has become totally confused. It defrosts more than once/day, the interior temperatures read >40F, and the ice melts. Thermistors check OK, heater resistance is OK. I replaced the defrost thermostat - no improvement.