1972 Road Runner 440: Had been running perfectly in 75-90 degree temperatures. No issues. Oil pressure right in the middle of the gauge, water temperature in the middle to slightly to the left, cool side. No problems of temperature gauge going to the hot side at stop signs or traffic lights. However after pulling into the garage and shutting it off, about 20 to 30 seconds later the coolant pukes out of the overflow tube on to the garage floor. About a half a cup of coolant and then it stops. Going back into the car and turning on the ignition shows that the water temperature gauge is pegged in the red zone all the way to the right. The car had a major service in 2018, new coolant, new radiator cap, new thermostat, radiator taken out and cleaned. All hoses are 5 years or less. This problem just showed up the last three times I took the car out, otherwise for two years, no issues. Any ideas as to where to start to fix it?