Over heating while stoped in traffic

I have read others questions and they sound the same over heating issues except that when I park it leaks a bit of coolant, but the fan keep working as it kicks on and thw car cools down. This does not happen every time I use the car only sometimes. And we have seen the coolant moves in the radiator.so I don't feel it is the fan.

A slow leak could be head or intake gaskets seeping or the plastic elbow leading to the heater core preparing to let go. Could also be a worn out water pump leaking some fluid around the shaft or a cracked radiator that only leaks when the pressure builds in the system. Is it actually overheating when you're stopped, or just heating up to the point the thermostat opens? If its an overheating problem it will typically keep getting hotter and hotter. In the short term make sure the cooling system is full and doesn't have air trapped in it. There's a bleeder valve near the thermostat for releasing trapped air. You do need to find the actual source of the leak though and get it fixed. It won't go away. If its seeping gaskets put a couple of units of Bars Leak in the system to see if that "fixes" it. If its hoses, water pump or radiator then you'll need to do some repairs.