Radiator

Check your coolant/antifreeze level in the radiator. Also, turn your heater all the way up and see if only cold or ambient (not hot) air comes out. This is a certain sign of low coolant. Not uncommon for a car to "cool off" while in motion because the forced air helps cool it, but starts heating up while at idle. Of course, you'll want to know where the coolant is going, so check all hoses and make sure to keep your radiator topped off and bleed the air from it. If it still wants to overheat you have either a bad thermostat, bad water pump, possibly a bad temp sensr, or some kind of obstruction in the system (like a clogged artery) that keeps it from circulating.