Howdy, thank you Davek0974 for your thoughts.
you say "At this point you have TWO systems of grounding - the control side (PC,USB,UC300) and the power side (BOB, MOTOR CONTROLLERS, MOTORS)"
and I totally agree, this is precisely why I was reluctant (opposed) to connect these two shields, as I tried to describe above. I fought for many years with ground loop issues on other type of equipment, so I became cautious if not maniac ...
Overnight I tried to acquire some statistic for the failures, with a reduced chain: (PC / UC300 / BoB connected and running).
Occurrences are random, or at least I cannot distinguish any pattern. The number of failures remains too large for any practical purposes.
Thus, I moved to another battle field, my old enemy, the PC ...
I realized that, for some reasons, TWO plugins for the UC300 were listed in the Plugin Config menu of Mach3. This state of affair may be due to my earlier attempts to up-date from a 2015 driver from Balazs, which failed.
Trashed the oldest plugin file, reboot, restart Mach3, reconfig, and just started with long term tests using the reduced chain without the second "circle" as you nicely describe it.
I'll inform about the result soon.
Enjoy you Sunday and thank you for the input. Philippe
I