JD, my agency runs almost exclusively JD 650J for fire attack, and we have been dealing with overheating issues since we first placed them in service 6 years ago
we are experiencing the overheating sometimes within an hour of operation (bearing in mind the heat and humidity, the fact that we are in close proximity to a wildland fire, and pulling a trail type fire plow through heavy underbrush)
blow out your radiator (we drop the grille and blow inwards, the run up the engines and blast it all out). We're in the midst of changing all our coolant to tropical grade antifreeze, removing the sand screen (between the radiator and fan) and a thermal wrap around the muffler. checking and blowing out or replacing your air filter regularly may help as well