I may have missed it in the details...but why not use the fiber that
was working fine?

No easy way to completely rule out grounding, or to rule out a problem
with the circuitry driving and reading the co-ax, but if these
problems go away using fiber, seems that's the ticket....

No reason, really. The optical requires the use of an external USB device.
If the coax worked, I'd just as soon eliminate that extra device. But it
brought about a puzzling finding which I couldn't explain.