I love the toughies . I gotta stick with the basics: not enough water flow or not enough airflow. I'd make sure there is adequate airflow thru the rad however way you choose to check that. I'd toss the stat for an hour test which wont hurt the eng it being gone for an hour. I'd double check that lower hose/spring & you could block the rods up so it's on the power circuit just to elim AF ratio/potential vac leak. You might pressure check the cooling system when hot to see if there is a minor pinhole leaking combustion into the coolant. Keep us posted! With it going up to 240 there has to be a severe coolant or airflow restriction showing up at higher RPM when the eng is producing the most heat & that is when the inadequacy shows up. I have the same vehicle ex it's 2wd. You wouldn't happen to have a shop manual or a wiring diagram for it would you? Gas gauge is non op