Freezer on top of fridge. Both the fridge and freezer get warm. Items in the freezer freeze and thaw, then freeze and thaw.

I believe it's defrosting too long.

I've pulled out the defrost thermostat and tested it for continuity. At frozen (not sure of the temp) it has continuity. At room temperature it doesn't. I don't think my model has a defrost timer, but I could be wrong. I didn't see a timer in the part list. The compressor kicks on and things get frozen so I don't think it's the compressor or the run capacitor.

Is it the defrost thermostat? Does the defrost thermostat in this model control both when the defrost heater comes on and when it shuts off? Could it be the temperature selector knob located in the fridge? Does my fridge have a defrost timer? Could it be the run capacitor?

Thanks for any help,
Jared