Is the fan cutting in?
Thermostat would usually cause it to boil over under all conditions as it bypasses the radiator, if it cools down when driving its not that.
Sounds like the thermo switch for the fan, Look down the right hand side of the radiator (i think) might be ther other side on yours, there is a sensor threaded in, near the bottom, disconnect it, bridge it with a bit of wire so the fan constantly runs, idle the car for a while and see if she stays cool! I recommend idling it untill it start to over heat, bridge it then idle again, you will see a clear diffrence.
If that fixes the problem, its time for a new thermo switch!
