My car overheated last night at highway temperatures, i noticed a puddle of coolant on the ground under the car and the radiator overflow cap had an immense amount of pressure. I checked the coolant overflow tube and coolant is flowing in there (waterpump) heat works very well, not sure what caused this any suggestions? (Car is holding coolant now and has been test driven 10 kms on highway speeds. temperature stabalizes at 90 degrees and doesn't rise (with the exception of the overheat.