I have a 99 911. It overheats driving on side street but not on the freeway. I replaced the thermostat already. Water comes out from overflow reservoir?

Overheating can be caused by anything that decreases the cooling system's ability to absorb, transport and dissipate heat so heres what will cause overheating A low coolant level, a coolant leak (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn't open but you said u replaced it, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller, or even a defective radiator cap. So i would go down the list of things and see what you can find. Thats all the things that cause overhteating,now its up to you for process of elimination. good luck and let me know what you find. Overheating can be caused by anything that decreases the cooling system's ability to absorb, transport and dissipate heat so heres what will cause overheating A low coolant level, a coolant leak (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn't open but you said u replaced it, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller, or even a defective radiator cap. So i would go down the list of things and see what you can find. Thats all the things that cause overhteating,now its up to you for process of elimination. good luck and let me know what you find.