Sounds to me like the sudden max temp was caused by cavitation around the probe and when you dropped the rpm the cavitation ceased and the temp dropped to the normal range. First thing I'd do is flush the entire system, replace the radiator cap (in fact I replace mine every other oil change because the spring inside gets weak and won't hold static pressure and caps are CHEAP and I'd install a real temp gauge not the OEM one.
One of the most neglected things on any tow vehicle (besides the transmission fluid) is the radiator cap.
