[shorod]

Is the cooling fan working? Before tackling the thermostat, make sure the cooling fan works properly. If not, a new thermostat won't fix your issue. When you're traveling at highway speeds, the air is moving through the radiator on its own, keeping the cooling system cool. As you slow and get into city driving, the cooling fan should kick in to pull air through the radiator.

Also, test or replace the radiator cap to make sure it is properly pressurizing the cooling system. Pressure on the system will increase the boiling point (and freezing point) of the coolant. It doesn't really sound like you have a problem with it boiling over, but the pressure cap is often overlooked.

-Rod