Refrigerator keeps frosting up and not cooling

Refrigerator started frosting up and running all the time.I'd defrost it and it would frost up again in about a week.The timer seems to be work so I just guest it mite be the Defrost Thermostat.I removed the duck in the center and back panel in the freezer compartment defrosted it and switched Thermostats and replaced panel and duck, going to lower compartment of the refrigerator.So far it seems to be working a little more time while tell.