Which thermistor and thermistor table have you got selected?
The fact that it's jumping around could mean that you've got the wrong table selected, or that the table is not calibrated well (gaps between points are too high at the higher temps).
One thing that can definitely cause temp problems is if you have any sort of electrical connection between the hot end element (resistor or cartridge type heater) and the thermistor leads. Specifically, some of the ceramic glues that tend to get used on hot ends can conduct enough to throw off readings (or in severe cases, enough to fry the thermistor input).
As such, I tend to avoid covering the ends of the resistor with the ceramic glue I have (which was from MakerGear) so as to avoid this issue, and I'm very careful with the thermistor as well. If either are held in with a grub screw instead of glue, just make sure to not over-tighten them, particularly the thermistor.
If the thermistor cracks, it'll most likely fail 100%, but I've not seen a lot of failure modes of thermistors to be able to comment on that.
Note: I've done a bit of testing of some of the ceramic glues (active component is usually sodium silicate), and this effect lessens over time and use, but it never truly goes away. And while the resistance may be something of the order 10Mohm, this is still a reasonable amount compared to the average 100k of a thermistor, and worse if you take into account that the hot end heater runs (usually) off 12V.
The fact that it's jumping around could mean that you've got the wrong table selected, or that the table is not calibrated well (gaps between points are too high at the higher temps).
One thing that can definitely cause temp problems is if you have any sort of electrical connection between the hot end element (resistor or cartridge type heater) and the thermistor leads. Specifically, some of the ceramic glues that tend to get used on hot ends can conduct enough to throw off readings (or in severe cases, enough to fry the thermistor input).
As such, I tend to avoid covering the ends of the resistor with the ceramic glue I have (which was from MakerGear) so as to avoid this issue, and I'm very careful with the thermistor as well. If either are held in with a grub screw instead of glue, just make sure to not over-tighten them, particularly the thermistor.
If the thermistor cracks, it'll most likely fail 100%, but I've not seen a lot of failure modes of thermistors to be able to comment on that.
Note: I've done a bit of testing of some of the ceramic glues (active component is usually sodium silicate), and this effect lessens over time and use, but it never truly goes away. And while the resistance may be something of the order 10Mohm, this is still a reasonable amount compared to the average 100k of a thermistor, and worse if you take into account that the hot end heater runs (usually) off 12V.