Well, it could be a bad thermosensor...

Bad Thermosensors do show themselves in multiple ways, among them fluctuating their readings before fully failing.

Thermosensors have ranges

Every thermosensor comes with a range for which it actually can operate reliably. For example, the TS2289 is rated for -40 to 90 °C, and thus would be not usable for a 3D printer at all, while TS2452 is rated for -100 to 600 °C.

The glass bead sensor on a Creality Ender3 is a ‎ntc 100k thermistor. That is only a class, and it contains multiple entries. This is not actually a specific sensor but a class that contains multiple sensors. For example, NTC 3950 is listed as -40 to 300 °C, which means it might be barely useable in that area, yet there might be larger fluctuations than normal as the resolution at the ends of the measuring range gets higher. In about the same dimensions, you might however also get a very similar sensor like thos from the Datasheet of the NTC Type BR Series. This document lists BR11/14/16/25 with a ceiling of 200 °C and B32/42/55 with a ceiling of 300 °C, all of which are in a very similar size range.

When it comes to sensor cartridge style sensors, both the PT100B or PT1000 are claimed to accurately work up to 450 °C and might operate reliably even at 500 °C. The standard E3D Thermosensor is a similar PT100, and would also operate fine in that range.

Simply put: to reliably print at those high temperatures, you might want to change the thermosensor for a type that actually does operate reliably in those high-temperature areas.

P.S.

^{I do not use the word thermistor (or thermocouple) here, because those describe specific types of sensor. The more generic term Thermosensor describes anything from a simple thermometer, over thermistors and thermocouples to an IR sensor.}

Well, it could be a bad thermosensor...

Bad Thermosensors do show themselves in multiple ways, among them fluctuating their readings before fully failing.

Thermosensors have ranges

Every thermosensor comes with a range for which it actually can operate reliably. For example, the TS2289 is rated for -40 to 90 °C, and thus would be not usable for a 3D printer at all, while TS2452 is rated for -100 to 600 °C.

The glass bead sensor on a Creality Ender3 is a ‎ntc 100k thermistor. That is only a class, and it contains multiple entries. This is not actually a specific sensor but a class that contains multiple sensors. For example, NTC 3950 is listed as -40 to 300 °C, which means it might be barely useable in that area, yet there might be larger fluctuations than normal as the resolution at the ends of the measuring range gets higher. In about the same dimensions, you might however also get a very similar sensor like thos from the Datasheet of the NTC Type BR Series. This document lists BR11/14/16/25 with a ceiling of 200 °C and B32/42/55 with a ceiling of 300 °C, all of which are in a very similar size range.

Not all sensors are glass-bead style, like the standard e3D v6 Thermosensor cartridge, which is listed as a Semitec 104NT thermistor the ceiling listed as 300 °C, but the same form factor also fits other cartridges. The sensor listed by e3D actually does come as a glass bead style too, listed as -50 to 300 °C

Such other sensors might be PT100B or PT1000 are claimed to accurately work up to 450 °C and might operate reliably even at 500 °C. A similar E3D Thermosensor is the PT100, and would also operate fine in that range.

Simply put: to reliably print at those high temperatures, you might want to change the thermosensor for a type that actually does operate reliably in those high-temperature areas.

P.S.

^{I do not use the word thermistor (or thermocouple) here, because those describe specific types of sensor. The more generic term Thermosensor describes anything from a simple thermometer, over thermistors and thermocouples to an IR sensor.}