cooling issue

After 45,000 miles I mistakenly thought my coolant issues were over. On a short drive the other night I noticed higher than normal water temps. Whenever that has happened in the past I normally just keep topping off the overflow tank as the engine cools and sucks the water up. However I didn't that night. Several days later I went to add water through the vent plug atop the thermostat housing. As I added about a cup of water, the overflow tube dumped about a gallon of water out. I bought and installed a new radiator cap and the problem continues. I thought the radiator cap sealed the surface below the overflow opening. I doubled up the gaskets and still no luck. So... any suggestions? Any and all suggestions and comments appreciated. Thanks Jimmy