Coolant Resovoir is boiling over.

I assume that there are no leaks anywhere and the radiator cap is in good condition. A bad radiator cap wil be indicated by the coolant being heard to boil just after the engine is stopped because of the retained heat in the engine when there is no coolant circulation. Ensure all the hoses and clamps where they connect to the various pipes in the engine bay and fix/replace any that are suspect. The heater hoses and those pesky little bypass hoses in the cooling system tend to go hard and split if they are over 10 years old, allowing coolant loss. Those spring type hose clamps tend to be less effective in clamping as the hoses become hard. Worm drive ones are the best to use.

If there are no leaks and water pump and thermostat are working correctly, but undetected loss of coolant once the engine heats up, then it could indicate a problem with the head gasket (Usually caused by allowing engine to become excessively overheated when cooling system has run dry and warping the head) allowing very hot high pressure combustion gases into the engine water jacket, which super heats the coolant in the engine block, which then boils off, and is released via the cap. If the head gasket leak is bad enough it will allow coolant into the combustion chambers which will produce white steamy smoke in the exhaust and allow coolant to get into the sump while the engine is stopped. This is indicated by a white oil and water emulsion on the dipstick.