00 Malibu Boiling over from Res Tube

I have a 2000 Malibu, had it since 2004, about 4 yrs ago (06) I changed the water pump due to a leak from the weep hole on the old pump. I did not change anything else at the time. here in the last probably 2 months I have started to notice that when I arrive at where I am going I have a large puddle of coolant and am having to replace quite a bit of fluid.
I have gone to mechanics and have gotten several different answers, from my head gasket is bad, to the by pass hose and the thermostat. I almost changed the thermostat but when I looked further I noticed that the leak was coming from the Reservoir Tank over flow. I filled the tank and was able to get my car home with out a problem.
When the car gets to operating temp. it runs normal, doesn't get any hotter and cools off when it hits the correct temp. so That leads me to believe that the thermostat is still good.
I finally noticed the other day from driving my wifes vehicle that the radiator fan in my car does not come on at all.
Running the car on the highway, there is a lot of natural air flow going through the radiator which I am believing is keeping the car at normal temp. It is only when I get to the stop and go traffic of the city streets and finally to my job that I see the leak. Could the fan not working cause this to happen? or would it be something more?

Thanks in Advance

Since you said the engine temperature is normal, maybe your reservoir cap is just not holding pressure. Try replacing it and see if that helps. Also with the cap off and engine running do you have any pressure building up like from a blown head gasket?

Auto Zone, Advance and Pep Boys have loaner pressure testers. You need to use a hose clamp and bolt to pressurize the system in the vent hose. Put about 10 psi on the system with the engine running if the gauge pulsates you need head gaskets.
Try the previous suggestion first.