If you can run at normal temperature on the highway, the thermostat is open. When it runs hot at stop lights, if it cools down a minute or two after you raise engine speed in neutral, the water pump or a loose belt are suspect. Since the water pump is relatively new, a loose or eaten away impeller are not likely but a loose belt would be more common. Typically you'll also get a belt squeal, particularly when turning at slow speeds or when stopped, and you'll have weak or pulsing power steering assist.
Since the radiator was replaced, when the fan turns on, check that air is blowing the right way, toward the engine. I've seen stranger things before. Someone could have repaired the electrical connector and switched the two wires around. I know that sounds silly but it just takes a second to check.
Also look for a missing rubber gasket under the front and rear of the hood. The front one seals air from going through that gap and forces it to go through the radiator. That is more of a problem at low speeds when the air flow relies on the fan.
Wednesday, June 27th, 2012 AT 9:05 PM