So interesting. I remember parts of this but am just getting around to asking. So my j series got an upgraded radiator. Temp tops out like you said, but then dips down. We were trying to sort it. I think we replaced the thermostat a final time and done with it. So it was not as detailed a solution. I stopped addressing it when it stopped getting too hot on the top end. It still dips though. What should my concern level be given the temp variation, many times per day? Exactly what should I be concerned about, that is, what else could breakdown in what order? Very cool to know the thermostat on one end and the other on the other end, as a cause, and the thermo opening as another cause. No clue how to figure that out on mine, yet, but its in the hopper for future sorting out