Originally Posted by Vin
when was the last time you had the cooling system serviced? If it's only happening when you are stopped, you may have air in the system.
Good point - if this is the case - you could run it with the radiator cap off until hot (yes it will overflow so find something to catch it with) and then continue to add fluid after see the thermostat kick in (usually when the fan kicks in) and the water level drop rapidly. After a few cycles of this - the air should be out of the system.
BUT CAUTION - only take the cap off while cold and not under pressure - if you wait till warmed up the results will not be good.
Also I would advise having someone keeping an eye on the temp gauge - if it goes too far past the middle and the thermostat has not opened, well then you know what the issue is. (Turn it off, let cool down, and replace the thermostat)
Hope this helps