For the rest of the community...

I have down graded Shopbot control software to version 3.8.46 (and reloaded the controlbox firmware for that version) and everything is back to normal.

This would rules out CNC problem and almost rules out problem with my computer itself (fairly powerful and stable).

I have brought my problem with the tech support at shopbottools and here is the situation for now: they are aware of controlbox firmware issues with some users and they are working on it. For now I will keep the older software and finish my current project.

They suggested maybe I could try installing the beta 3.8.7, update firmware, then reinstalling 3.8.6 without installing firmware - in other words : using shopbot 3.8.6 with the firmware from the beta 3.8.7 to see what happens.

Thanks you all for your support. The forum contribution is greatly appreciated.

Marc