98 Sunfire cooling issue

I have a 98 sunfire I have been having some cooling issues that I have been trying to track down. But first this is what I have done so far:

replaced water pump 2 years ago
just replaced thermostat
just replaced radiator with 1" core

and im still having some issues if your driving down the road it runs just fine no running hot or anything but as soon as I stop and sit for a little bit it want to run hot I know the fan motor is good because when I turn on the ac the fan kicks on I just think that the fan is not kicking on when the thermostat opens up. So I just want to know what you guys think do I need to start looking into the fan switch (which I don't know where its at) or something else. Oh as a side not before I replaced the radiator I did notice the coolant was a orange/brown color if that helps