Runs hot when stopped, but cools back down while driving.

Have you checked if the radiator fan is turning on? When moving, you have natural air flow through the radiator. Since that cools it down, you know all the other parts are okay and working.

Catalytic converter is in the exhaust system. It can promote engine overheating if it's plugged but the engine would have almost no power at all. You'd have a real hard time reaching 25 mph.

If the engine cools down, as you said, at higher speeds, you have insufficient air flow through the radiator at low speeds which is a fan problem, or you have insufficient coolant circulation which is a water pump or belt problem. If the engine gets too hot and the air from the heater is cool, suspect circulation trouble. The hot coolant is not getting to the heater core in the dash and to the radiator. At higher speeds the engine is running faster so the water pump is too. If you can carefully shift into neutral while driving on the highway and coasting down a long hill, with the engine idling, if it overheats while still at speed, it's poor circulation. You can do the opposite too. Put it in neutral while stopped and raise engine speed. The fan should come on. If it still runs hot, suspect poor circulation. If it doesn't get hot until you let engine speed come back to idle, suspect circulation trouble.

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.

Michaelrodgers, what's the verdict with this? Have you been able to determine what the problem was/is, or do we need to take it a little further?