I've been experimenting, and it doesn't appear to be any of the issues mentioned in your first two paragraphs, and the z drivers don't reachn unreasonable temperature, and pointing a fan at them doesn't seem to have had any effect. I don't currently have the ability to change motor currents, annoyingly.
Travel between two points however does appear to be causing issues. Whether or not retraction is on doesn't appear to affect anything, but modifying the object in the pictures attached to remove unnecessarily long travels has significantly reduced the issue, but not removed it. I'm thinking this is because there is still some travel within the confines of the print.