[Richard]

Great post, you gave the detail needed to get good responses. I am sort of echoing what others have said.

First, use an IR gun on the thermostat housing to verify the temp is real. If you really are seeing 250 then the genset over temperature would have shut it down and coolant would be boiling over. You mentioned neither.

But if it is real, look carefully at the airflow and where it is sealed to force it through the radiator. Not going to comment on the loose rubber piece other than to say make sure the airflow path is as sealed as possible

As you drive the outside air pressure at the louvered door will be less than when stationary. In addition to making sure the air path is sealed, make sure the belt is tight from the motor to the blower.

One last thing that occurred with an owner many years ago was that if he cranked the genny while driving the motor would turn backwards. I never really understood that electrically but the result was the gennie would over heat and shutdown

Let us know what you feel find