HELP! My 68 Riviera 430 intermittently sprays coolant all over the underside of the hood. The system was pressure tested, and it holds pressure, and there are NO leaks. New radiator 10 yrs ago, new hoses, new water pump 10 yrs ago, new fan clutch 15 yrs ago, new radiator cap one month ago. I drove the 12 miles at 60 mph (all down hill -from 6800 ft. to 5200 ft.) to fill the fuel tank. When I entered the gas station it looked like smoke from under the hood. It was coolant that had gotten on the exhaust manifold. The hoses were NOT pressurized and the temp. gauge never went past the half-way mark. Filled the tank, added coolant and drove around town and then home - all up hill from 5200ft. to 6800 ft. Temperature remained normal. Sometimes the coolant is sprayed and sometimes it isn't. Where do I look next??