Super bizarre…

Any chance there’s some mechanical damage to the thermistor? If you’ve rebuilt the hotend a bunch of times it’s really easy to do, I’ve totally reefed down on a setscrew before and flattened one, could lead to it possibly reporting a wrong value, a crack + high vibrations during printing + thermal cycling could lead to it getting worse over time. Sudden shifts to me would suggest a problem with the thermistor itself, especially given the total overhaul you’ve done. As crappy as it is too, if I recall electrical components often follow an early failure pattern where they can have a higher failure rate at beginning of life and then drops off, for the price of thermistor cartridges it’s not a bad idea to keep some on hand just in case.

To make your life easier, Do you have a molex connector at the hotend? A lot of thermistor cartridges come with short leads and a molex connector, makes swapping them so much easier. If you don’t already have some crimpers, Engineer PA-09 is a solid pair that’ll do everything from molex to jst.