@dc42 The chamber heaters do bounce around when heating up initially from room temp. To compensate, we've dialed our M307 back to next to nothing. As for the the probe, it's just a run-of-the-mill B3950 thermistor.
M307 H2 B1 C1800 D350 R0.02
The temperature can and does swing negative while initially heating. Some of these faults may be due to momentarily opening the doors to the chamber while heating, but more often than not, the faults occur when the printers have not been touched (and the doors are closed).
We understand the importance of faulting a heater when it's too slow to rise (ex: leaving the doors open), but really would like to see the parameters relaxed while initially heating.. unless there is something more that you can suggest with our M307?