Engine overheating?

My 1999 Cadillac overheats but only when it sits not when you're driving it has absolutely no coolant leaks when you are driving the car is fine but when you put it in park and the car is still ruining coolant rushes out of the overflow hose I don't know if that would cause the car to overheat I just had an oil change and the oil was looking like oil not like there was water mixed with oil and the cooling does not leak out and I just replaced the radio pleas help me to know what to do the car does not smoke or hesitate when you put the pedal down I like the car and would like to keep it but I need to know if it's going to be too much work for me and if so I'm going to have to sell it