Question: Erratic, Unstable, Jerky Trackpad Cursor - Update - Solved
I logged my previous post of the same name as solved due to a faulty third party power adapter but this was incorrect. I'm unable to update that post so I will place the actual solution here for reference.
The erratic trackpad cursor was due in my case to the standalone plug that fits onto the power adapter. I was able to reproduce and to solve the problem at will by fitting the standalone plug to power adapter and then swapping it and fitting the extension cable into the adapter instead.
The problem recurred with each of the three standalone plugs that I tried and each time was solved when I fitted the extension cable instead. It's still possible the problem is in the third party adapter connection rather than the standalone plug but what the case it is solved when the extension cable is used instead.