I cannot be certain, but this may be a case of move commands while in either gcode mode of g90 or g91, while expecting the other mode.
If you google “gcode g90 vs g91” you get a quick help note that says it this way:
These G codes change the way the axis commands are interpreted. Axes commands following a G90 will move the axes to the machine coordinate. Axes commands following a G91 will move the axis that distance from the current point .
Put another way, if you are in G90 mode, and you give a command to go Z -1, you are saying to go to the machine coordinate of Z = -1, not telling the machine to move down -1 from where it is currently.
Come to think of it, this business of G90 versus G91, may explain why @JW1 was having strange results with using Repetier host.
@JW1 can you confirm which of the two modes you were in, and that you were tracking the difference between “absolute coordinate” commands versus “relative to where the machine is now” commands?
NOTE: I have not ever used Repetier host, and so I may be off base, but it’s worth mentioning.
