Radiator intact, coolant level fine... car on the verge of overheating.

What is your question?

Hi, My car started to overheat last week because I hadn't checked the coolant level in a while. I went ahead and filled the radiator and reservoir up per the instructions... I'm out driving on a cool morning (about 60 degrees Fahrenheit and foggy) today and the engine started smoking (again) and the temperature needle went WAY high. Assuming nothing's leaking and that the proportion of coolant to water is correct (I'd have bought it mixed, but someone in the house was paranoid that our indoor cat would get into it somehow... don't ask... the only "pet safe" stuff I could find required dilution), what else could be going on and what might repairing it cost??