Dart over heating???

I would also try running a 160° thermostat. You don't need the heat in the summer.

I also agree on getting a good shroud.

You need to concentrate on the airflow at idle/standing still. If you run ok when moving, your system has the capacity to cool, it is then getting restricted when standing still due to low airflow through the radiator. From what I'm reading here, you are not getting enough airflow through the radiator at idle/standing still. Get a good fan 5 - 7 blades and shoot for half of it to be covered in the shroud and half out.