This could be a couple of different things, either the thermostat is stuck open not giving the coolant enough time in the radiator to cool down, or your fan is not operating properly.
It sounds like your engine fan is not starting when it's supposed to. Open the hood, start you engine, let the car run. The fan next to the radiator should come on before your temperature sensor gets into the hot zone.
More than likley, your cooling fans are not coming on when they are supposed to. You might neeed to replace your cooling fans, or possibly just your cooling fan relays.
It sounds like you are having a problem with the coolant getting to the radiator when you are idling. You can try to top off your coolant level and if that does not work try putting a thermostat in it. You should be ok with a complete radiator flush and thermostat.