the overheating at stop and low speeds could be the fan not running. find and disconnect the temperature sensor for the PCM. on my 3.8L it is on the rear center of the engine near the upper radiator hose. the fan should come on low for a few seconds then speed up to high speed. this checks to see if the the computer is capable of running the fan or not. by unhooking the sensor the computer goes into a high fan default mode. if it doesn't either the fan is bad, or the relay module. if it does come on at both speeds plug the sensor back in and let the engine warm up and see if it ever comes on on its own. if not it is probably the sensor. mine won't come on until it gets to the 2/3 position but i am having different issues with mine. hope that helps BTW, don't LEAVE the sensor unplugged. that is just for testing and emergency purposes and will kill the gas mileage | 
