2000 Cadillac Coupe DeVille DTS North Star keeps overheating and I don't know why. Replaced water pump, thermostat, misc. gaskets, no detectable leaks. Added internal engine sealer twice. Car consumes 5 quarts of coolant / water after driving for 10 minutes. Not sure where water is evaporating to. Coolant overflow bottle empty after overheating. No issues of overheating during idling, only under load. Please advise.