I'm just hoping you don't have a head gasket leak.
You can try to use a 70% coolant and 30% water mixture for higher temp protection as well as lower temp protection.
I would however remove the two coolant temp sensors and clean them first the put them back and and then test it. One of the coolant sensor is usually on radiator itself and the other one is behind/below the throttle body intake.
When you fill the cooling system, make sure you turn the bleeder screw above the thermostat and open it until coolant is flowing out of it without any bubbles.
You can rig the fan with a manual switch to turn it own while on a traffic stop and go.
Check this link
http://www.thirdgen.org/tech/cool8dual.pdf
http://www.babcox.com/editorial/tr/tr110046.htm
Check this link about installing a manual fan swtich. It will give you an idea how to do it on your vehicle.
http://www.v6z24.com/howto/fan