Car overheated twice after a 4000+ foot climb. Once over pass it cooled. Water was low and water apparent on fan, and around radiator. Took to dealer. They could not overheat. Next time I took in where water was present. They find no leak. Pressure and leak tests are good. Radiator, thermostat, hoses only 18 months old. Their best guess is partially blown head gasket. Does that sound reasonable? They say water is not properly circulating and it spits the fluid out overflow cap. What should I do? Drive until it overheats consistently? It does not overheat on normal 20 mile commute or around town. Need some advice. Thank you.