@dc42 - I've set M929 but this doesn't show any more detail - just that it's out of range, not that it's above or below the setpoint.
After ~10 hrs of printing I hadn't seen the issue, and then I saw it a few times in a row, within minutes. I'm used to a rock-solid temperature, and now I'm seeing something very noisy:
When the extruder is not moving, the value is nicely consistent:
I re-calibrated the extruder heater but the values were pretty similar, and now, watching the PanelDue, I see the temp drop ~20C from the setpoint nearly instantaneously, then jump back up to setpoint. When heating from cool (as in the inline pic above), I don't see this (it would take many seconds to change 15 degress!), so I really have no idea how such fast changes are even possible.
The part I'm printing now is just a cylinder and it's seeing huge noise:
I'm wondering if I have a failing thermistor or heater connection somewhere... it just doesn't seem like the temp measurement is trustworthy. The fairly weak extruder fan I have now seems unlikely to cause such a temp drop.
As always, any help would be appreciated.