I have seen several cases of engines overheating when a hose gets old and collapses. This will only show up when the engine is running high idle, as that is when the water pump has the most suction. Also the bypass hose on some systems will get plugged and fail or it's fittings. Have you owned the combine since new or did the previous owners not keep the filter(if it has one)changed. And the Antifreeze should have been_be changed every 2 or 3 yrs to keep corrosion out of the system which can block passages. A cracked head will also cause it to overheat but you have probably check that with the pressure test. Now, since you have 2 of the same machine, do both have the same motor 670HI or 670IIJ This can be easily seen by the injector pump, Roosamaster(670HI) or Ambac(670I). If they are the same, do they seem to have the same power, if there is a difference in power output then you have a fuel problem(pump or nozzles)or timing, this will also cause it to overheat. Good luck!