Blew the board out thoroughly with compressed air. Still had the problem.
Going with the vibration theory, I started disconnecting the shapeoko stepper by stepper looking for a loose connection. Wound up with the tinyg on the floor next to the shapeoko table, totally disconnected from the shapeoko and still experiencing the issue.
Then Alden and Riley at Synthetos suggested I put the proxxon on a separate A.C. circuit from the tinyg and computer. This seems to have fixed it!
Now I've got it back on the same circuit (for practical reasons) with ferrite chokes on the proxxon power cable and the u.s.b. cable and it seems to be 90% fixed (I am getting a fail once on every three runs).