I am trying to service So-Low model# U85-18( -40to-85DegC). It starts up normally, and both cascade compressors (Copeland RF41C1E-CAA-210) run normally until reaching target temp, which I have set at -60 Deg C. They they then shut down normally at target temp. When deadband ends, and refrigeration system attempts to start, unit blows 20Amp breaker. As I am not there when it occurs, it has been a bit difficult to predict when it will fail. Unit runs on 115V,1Ph,60Hz. I am beginning to suspect either potential relays or strt capacitors, except for the fact that it runs normally when started from scratch after several hour shutdown.