In the dunes you are using a lot more of the available power, try bushing your rail on the street 50 ft, easy, now try it in the sand, now up hill in the sand.
airflow is definitely the problem, anything you can do to make an improvement will help but I'm sure running at 106 degrees does not help.
Maybe you could build an auxiliary radiator to clamp onto the frame when you are in the dunes, then remove it when you don't need it?