@dc42 could M557 show available Wi-Fi networks (plus signal strength) as well as remembered?
We would need to have a separate command (M552 S3?) to do a network scan and then return the results. It involves changes to the wifi server firmware as well as RRF (and PanelDue if you want to display it there). I'll add it to the work list.
Well, I took the plunge and updated to firmware 3.1.1
A few things were a royal pain, things that I'd tuned that needed finding and re-entering. Like extruder steps that were entirely wrong. There didn't seem to be a spot in the configurator for using 2 motor drives for the Z axis, Pressure advance needed tuning, and it scared the crap outta me when it said that the heat bed was faulting. (PID tuning needed to happen again.) All that stuff I figured out 2 years ago and have since forgotten.
@hamiltonprops From re- reading your OP, you say that the temperature drops 20 degrees within a second or two. That would indicate it's not a "real" drop in temperature but rather a drop in the indicated (measured) temperature - unless your hot end has an insanely low thermal mass. So, yes I go for some sort of cross talk or electrical "noise". Are you sure you haven't got a bad crimp on the thermistor wiring somewhere?
If you use long wires, or you will have connectors in the cable, then 4 wires is best. If the cable is short with no connectors except where you connect it to the PT100 and the daughter board, then 2 wires is sufficient.
As you only have one heater, try putting this in your start gcode
G10 P0 S205 R205
G10 P1 S205 R205 (or whatever temperature you want to print at). That'll set the active and standby temperatures the same so you can instantly switch between tools.
Then make sure that Cura does nothing other than select a tool by putting in a "Tn" command (where "n" will be either 0 or 1). Oh, and make sure that there is nothing in your tool change files - at least for now.
In 10 hrs of printing since adding grounding the extruder motor body (on a PG35L), with two prints, I haven't seen this issue recur yet. This isn't proof of a fix, but it's highly encouraging, since the MTBF seemed on the order of a few hours previously.
I used a wire that is tucked between the outer shell and the heatsink, and connected to the power supply ground.
Excited to be able to try other stuff now - updating to Duet firmware 2.0, trying out other slicers, etc!