Wondering what I am doing wrong.

Trying to loosen up the sand on our beach with a pull behind harrow. Nothing to big and my sportsman 500 efi pulls it fine. My problem is it keeps over heating on me.
I originally was pulling in Low around 3500- 4000 RPM and it over heated, I tried in high gear around the same 3500-4500 RPM thinking the speed would but more air across the rad but again it overheated.

Am I just trying to go too fast?