That's a thought Dave - thank you - I think that may work.
De- focusing a little may also have the same result of smoothing things out a little I suppose but it has become a real challenge now and I have just got to find the actual cause of this phenomenon.
It is so strange that, in the laser world, everybody seems to know about it, nobody admits to it and the cause has just not been defined. If this was the plot for a book it could be a best seller.
So far, I have checked my Y axis for backlash / slack and also for unequal step size (looking at the line scan spacing under magnification shows no problem). I have measured the time taken to complete each line (feedrate) from the X axis direction signal and they are as equal as I can measure on a chart recorder. I have recorded the incoming mains voltage and there is some fluctuation, but not much, certainly not in steps like the images show however I have a UPS voltage conditioner coming for a trial. And finally, for the test sample, I have matched the vertical image size and stepover exactly to the number of vertical pixels.
The saga continues.
Tweakie.