[Sam Bozzette]

To cwilson, the issue may not be the radiator, it may be the fan mounted directly to the radiator, if yours is mounted that way? If you look at the total surfae area of the radiator then look at the fan you can see it is not pulling air through a good portion of the cooling fins. so when the car is not moving this portion of the raditor is not doing any cooling. Sorry for being so long winded. In short you need a raditor shroud with the fan mounted at the opening so it pulls air through all the cooling surface.
Just my two cents, Sam