Hi @rgmcleod52, did you ever do any further testing or was your issue resolved by the rebuild/maintenance you did? I am experiencing a similar issue, and like @Spanners alluded to, my hypothesis is that it is related to the Seam and potentially limitations of the smallest step the SM extruders can handle, rather than the filament or a potential wobble. I have done a more detailed write up of what I suspect which can be seen here if you are interested - Any advice on strange issue? - #8 by Waterboy, however this is only a theory and when I stumbled on your thread I thought to check in if it was resolved noting that it happened approx. a month ago.
Kind regards,