1990 buick lesabre, radiator fluid overflowing out of cap?

there should be enough fluid in the radiator to cover the coils, if you can see the coils the fluid is too low, I would have the thermostat changed and the system flushed out and see if that helps and if it doesn't I would be looking at the radiator as being plugged up or possibly the water pump, my bet would be the radiator as old as the car is.