I apologize if this has already been mentioned but the radiator could be stopped up one of two ways.
First the core itself could be stopped up. If this is the case one section of the radiator will be much cooler than the rest. This will be the section that is stopped up. You can do this by removing the fan and feeling the radiator or use the non-burning of the skin method with an infra-red temp sensor. Keep in mind that if the radiator is plugged in the area of the fan clutch the clutch will NEVER engage because it has cool air blowing across it. This sounds like your problem. Realistically you can remove the fan and the cooling system should still function properly. Meaning that if you are sitting in traffic and the engine temp climbs once you begin to move again the air alone passing through the radiator should begin to lower the temperature. If the temp remains the same or climbs than you have a fluid or air flow problem which brings me to the second possibility- air flow.
If the fins on the A/C condensor or radiator are clogged with debris (bugs, dirt, etc) then the inadequate air flow will cause the same problem. These must be cleaned!
And a tip: Next time it overheats while driving, turn the heat on with max blower. After all the heater core is a small radiator.
Hope this helps!
