1997 Pontiac Sunfire coolant leaks out of cap

1997 pontiac sun fire.only in the summer months for the past three years when i stop and shut the car off water or coolant will leak out of the cap on the overflow/ filler bottle on the firewall.the car does not over heat and the thermostat has been changed along with the water pump.the temperature gauge on the dash shows a normal reading.i have also replaced the cap on the filler neck. it leaks to the point where the container has no fluid left in it. could it be a possibility that the cap is not a o.e.m. part? the odd thing is during the winter there is no leakage at all. any input on this problem would be greatly appreciated.
thank you, Adam Charles