A few thoughts:
- I assume the thermistor has well contact to the heater block and is in good condition?
- Assuming you run on 24V, the heater cartridge is rated to that voltage?
You may vary the value of this in [extruder]
#smooth_time: 1.0
# A time value (in seconds) over which temperature measurements will
# be smoothed to reduce the impact of measurement noise. The default
# is 1 seconds.