Get to the basics, get all the lubes and coolant levels checked, if not take it to a merc. mechanic... Could be because of thermostat, bad radiator, leakage in water body, or just a weak engine...usually when the car is driven due to the wind flow from front grill cools it off but when its static its the fan which which keeps it at normal temprature and water pump/body circulates the coolant...
Just the hot summers kicking in... t/c AH... A.