Dies and won't start

1986 Pontiac Paresienne Brougham started fine drove four blocks died and would not do anything. No power anywhere no lights no click just as if there was not a battery. Checked all wireing connections, engine started just fine drove three blocks did the same thing. Now can't do anything. When I pect on the black box on the right rear of the valve cover the engine will start then die dont know what it is may be part of the cruise. The car has 149000 mile and the large V8