Temp won't go below 42°F

True Refrigeration, Model T-23. Refrigerant is R-134a. Ambient room temp 80-85°F.

I purchased this unit used from a restaurant in the British Virgin Islands. Replaced shredded door seal. Wire brushed and vacuumed condenser coils and fins. Operated at 67°F for one month ( using as fermentation chamber for brewing). Held exact temp, Compressor ran occasionally (once per hr for 3 mins or less). Set temp at 35°F, would only drop to 47°. Reset controller for 43°F and fluctuates from 43-45°. Won't go any lower. Johnson controls probe and analog thermostat inside agree. Help!