Is it possible that there is some sort of electrical interference/grounding issue?
Anything is possible but if it was electrical intererence that would cause a loss of position and that is not apparent in your pictures.
In my opinion, everything points to flex in your cutting tool / machine and if all the mechanics are really rigid then I would look for a loose grub screw on an axis drive, pulley, coupling, thrust bearing, etc. After that I would replace the spindle bearings.
Sorry but I am just about out of ideas now. You have an excellent machine there which is well worth fixing and when you do find the problem I am certain it will be something very simple to fix.
Tweakie.