@kivlow Please post your full config.g and the response to M122.
Generally, in order of most likely to least likely, spiky temperature readings are caused by:
intermittent wiring - loose connections, poor crimping, frayed wires, dry solder joints. It may only happen when the wiring moves, or when temperature changes. failing temperature sensor - may only show when at temperature Electrical interference - usually from heater or motor wiring running very close to the temperature sensor wiring. Generally shows when machine is operating Power supply issues - faulty or failing power supply, external power spikes (eg large/noisy dc motor on the same electrical circuit starting/stopping) Hardware issues - faulty Duet board. Firmware bug - least likely because this is heavily tested, and would be reported by other users.Ian