The "wet" in the grass baths the bottom of the chassis with cool water. The chassis is aluminum, the engine mounts are aluminum, so basically, you have a big water cooled heat sink mounted to the bottom of your engine. Not to mention the spray of water that is most certainly hitting your cooling head.
Run it up and down the street where it's dry and see what you get.
Wet grass really changes tune quite a bit with a pan chassis rig of any type.