My 78 Olds 350cui engine leaks from valve cover. water pump front tubes R new. how 2stop it leaking?

I've got a 78 Oldsmobile 350 cui (no diesel) engine in an 78 Cadillac Seville, that leaks from the drivers side valve cover. The water pump, the radiator, all rubber tubes in the front are already replaced. When I am driving the heating works well, but when standing in a jam or longer on a traffic light, the air goes cold, but when speeding up again it gets back to hot. What can it be, that leaks, can it be an O-Ring or a water line or the thermostat or its housing and how can I easily stop it leaking permanently? thx for all answers