Ok, common sense (not poking fun, helping u learn how to diagnose). If your driving your getting a consistant amount of air flow. When ur sitting you do not. So that is what the fan is for. Let ur car cool. Start it up, let it warm up. Keep an eye on it and make sure the fan comes on. If it does and your still experiencing over heating replace your theromostat and I dunno how often u change ur coolant, but change it out. Most do a 50/50 mix. I do a 70/30 with more water.

-Ant.