I downloaded the latest Lulzbot Cura, and updated the firmware for my Mini since it mentioned something about fan hardware related updates.
Now my prints have become problematic. layer delaminations and other print quality issues. I had not changed anything else from proven files (had printed same .gcode before the firmware update). I think I have uncovered what is going on: temperature stability on the extruder. See attached temperature graphs. I have never observed previously the temperature stability problem.
How can I switch back to the previous firmware?
(EDIT: changed post subject/title, removing “in latest firmware”)
Wow thats a heavy temperature drop!
Do you know if the fan speed was related to the drops? I would recommend to flash back to the version that was working for you, then contact Lulzbot to let them know they may have a bug in the latest release…
I found Mini v1 .hex at https://www.lulzbot.com/firmware-flashing-through-cura. I flashed it through Cura now and am running a print now.
In my first print after changing firmware, I am still seeing the temperature dips unfortunately. 
What else could be going on??
Are you re-run the old (prior version of Cura) gcode files or are you generating new gcode file with the new version of Cura? If they are the old gcode file before you updated Cura it may be something else then when you are using the older version of firmware.
These are the same, proven .gcode (generated using previous 18.03 Cura). They have been working without problems until recently. I started having problems after updating to the firmware that came with 18.04. The problem is still there after re-flashing to v1.hex from the link above.
Now, the extruder temperature on my Mini is not responding at all. .gcode print originally seemed to be able to control extruder temperature (was going to try to print Mini-extruder_idler_block_v1.4c.gcode from the service bulletin in ABS). cancelled that after I noticed temperature was not responding, and tried manual control via Cura and Octopi. now, the extruder does not heat up at all.
I guess I’ll contact support.
I noticed the same kind of temp graph on my heater bed some time ago.
It turns out that a wire was failing between the print head and the controller board. The continuous flexing caused one of the wires to finally break from fatigue. What you may be seeing, as I did, is the connection being broken and reconnected. It didn’t seem to follow a pattern with the movement of the bed, but then I didn’t really analyze it.
It was difficult to find the point in the wire where it failed because the covering insulation was not damaged at all. When I did locate it, I could see the darkening of the individual strands where it had been arcing as it failed.
I replaced the wires and the problem ended.
Cheers,
Gary
I also had this problem, resulting in a complete failure of the bed heater. The wiring for the bed heater was replaced in an RMA process. Ultimately, it looks like the replacement of the bed heater wiring may have loosened the adjacent extruder heater wiring on the electrical board.
Since I’ve reseat the wires in their posts on the electrical board (ended up doing this while in consulting with Lulzbot support) I have not had an extruder heater temperature control problem.
I would double check the tightness of the connector screws to make sure they are tight and compressing the wires in them. There is a number of amps of current going through then both and if not tight, will heat up and cause intermittent problems.
Great minds think alike, I guess. 
I had done exactly this , especially after I had noticed one of the wires had some of its insulator flattened and compressed under the screw. This must have been the source of my extruder temperature issues, since my Mini has been working great since.
