When you disconnect the Z coupling and raise the gantry, does it fall back down under it's own weight? Mine does even after I have adjusted out any play in the gantry guides (no lubricant applied - yet). This ensures the motor doesn't have to pull the gantry down which messes up the auto levelling. I printed out your top bearing bracket @Michael G and fitted today - not done a new print yet but I'll let you know how it goes when I do. Simon.

@Michael G No, not changed the Nut - appears OK with the new coupler.

Update: I've found the 'odd behaviour' happens when I turn the Octoprint (Pi) on before the printer and let it load. Example: If the printer is turned on and I select 'Auto Home' on the printer controller then: 1) Z axis raises about 10mm 2) the X & Y home (top right next to the Z axis) (clicking the stop switches) 3) X & Y move to the centre and the z-axis slowly lowers down - up - down If Octoprint is already running when the printer is powered on. Then auto home from the printer control panel does this: 1) z axis raises about 20mm 2) 2) the X & Y home (top right next to the Z axis) (clicking the stop switches) 3) X & Y try to move beyond their max movement (bottom left) and jam there. Just to confirm - no control actions sent via Octoprint (Pi) in the above test and the settings in Octoprint are as per the install instructions (300 x 200 x 200) It seems to work OK if Octoprint is powered on after the printer (Marlin) has loaded so that's my work around. Simon.

Example of print difference with metal Z-Axis coupling. You can see the oscillation in the original (in blue) - reprinted with the metal coupling (in black) - you can see the new coupling in the background.

Agree, a metal Z motor coupling makes a huge difference for a very small cost (£3.25 on ebay). After fitting I also removed the metal plate at the top of the Z axis screw as the screw holds itself straight without it.

Thought I would share my first print. This is just as it came off the printer and hasn't been cleaned up apart from removing the additional printed base layers. For a first print I'm pleased with the quality.