Well the reason I asked about the hotend was because of you're using the little glass bead version of the thermistor, and the hotend has the hole to feed through, I had an issue on an old Frankenstein ender where it wasn't touching the metal inside correctly... As it heated up, it would kind of move the thermistor away from the metal. I solved this by putting a DIY metal shroud on it that held it in the middle while touching the hotend all around. I also used thermal paste to make sure there was no gaps.