Why is my '03 Nissan Murano overheating while idling?

Sounds like the fan isn't kicking on. The reason why it cools off when you accelerate is because fresh air is getting pushed into the engine and its surroundings. If you ever get stuck in traffic and your car is overheating put the heat on full blast. The heat in cars work off the engine. That is why in the winter when you turn your car on the heat comes out cold, because the car isn't warm yet. So remember if you get stuck put your HEAT ON as high as it goes. This is pain in the summer. Do yourself a favor and get this checked ASAP.