Here is a list of things to check.
1: The electric fan isn't coming on as it should. On 99.99999% of the cars out there, when you turn the A/C on, the electric fan should come on. If it doesn't then either the fan is bad or the sensor that turns it on is bad.
2: Bad/stuck thermostat. The thermostat could be bad but allowing enough flow at highway speeds to cool the engine but not in traffic/low speed conditions.
3: Partially clogged radiator. Either the coolant flow internally or the fins of the A/C condensor could be blocked/smashed restricting the air flow.
4: Low coolant/not enough coolant to water ratio. Pure water boils at 212* F. A 50/50 coolant/water mix boils at something like 280* F
5: Bad water pump. The impellers could be worn badly after 160k miles of pumping coolant, thus reducing coolant flow.
6: Clog in the coolant passages in the head or the block. Self explanatory though not likely.
7: Clogged heater core. Sounds stupid but sometimes a clogged heater core can disrupt flow enough to make the car overheat at low speeds but not at highway speeds.
There's a good starting point. Good luck!
Warped (not twisted!)
"Pity for the guilty is treason to the innocent." - Richard Rahl
"Democracy is two wolves and a lamb voting on what to have for dinner.
Liberty is a well-armed lamb contesting the vote." -Benjamin Franklin