maybe check the temp of a liquid that has been in the box for a day or so.. it should be somewhere between 38-42F. If this is so it is cooling ok. If it is only shutting off when it goes into defrost then the cold control could be stuck on. If that not it after you cleaned the condensing as stated before, then its time to retire that box. It sounds like it doesnt owe you a dime. Good luck !!