I note you say you have tried many different CV settings to eliminate the jerking. My model did the same thing as yours in relation to jerking until I adjusted CVs 54 & 55 to the value of 10 which are the recommended settings that Kernow advise for a Loksound 4. Mine is fitted with a Bachmann 36-557 decoder (essentially an ESU Lokpilot) and those settings work with that as well. Not suggesting that you haven't already tried it but if you haven't, that may work. As for bridging the polyswitch, I would suggest total removal of the polyswitch and bridging the two points where it was soldered to the pcb with a short piece of wire.
I have indeed tried 54 and 55 set to 10 - thanks for the suggestion - these are the recommendations put out by SWD to Kernow. These were my first efforts at resolution a few weeks ago. I actually found higher values gave 'better' results.
I have now removed the polyswitch and replaced it with a strip of wire on the motherboard. Same as my video where I bridged it, the loco barely has enough power to move itself, but if you push it, the wheels do revolve. In the opposite direction, they do not turn at all.
This evening, I have disconnected the track pickup wires from the motherboard and the motor connections and have wired them into a temporary 8 pin socket so that I could fit one of my 8 pin plug Lokpilots and bypass the motherboard. This is effectively wiring the motor directly to the decoder.
Same results as before with no motion. The motherboard is not the issue.
Looking at the motherboard, the polyswitch is wired in series with the pickups ie between the track and the decoder, which means it has the potential to corrupt a DCC signal. However, since my direct connection removed the motherboard completely, I have now concluded that these motors are simply incompatible with ESU decoders. I can't believe that someone actually thought these motors were a good idea! I concede that they are great for those of a DC persuasion, but hopeless for those of us in the DCC pond.
I have also now dismantled the loco to see what space is available for the motor. It has some tiny brass fly-wheels fitted - what is the point of such small fly-wheels - they are so small that they probably have negligible effect. Another bad design in this loco.
I'm going to research for a suitable sized Mashima-type motor to replace the rubbish motor this loco is fitted with. There seems to be a reasonable amount of space and certainly extra space can be made with a small amount of diamond cutting of the chassis block. Although I question why I should be doing this on a model of this price!
I am of the opinion that Kernow have made a really poor choice of motor.