Hello Tim!
In your test grid "tests.pdf" how reliable is the green Succeed case? Is it always? If so the noise in your shop is exceeding the amount USB can withstand.
First be sure the computers are set up according to the black viper minimum configuration. No unnecessary background activity starving the usb port. (a possible culprit).
Two possible electrical remedy's. First is buy a usb isolator for the machine that is hanging up. Then ground issues between the computer and the shopbot cease to be a factor. USB isolators are not common, then neither are shopbots that freeze, you may have to order one off the internet, I dont know of any place in town here that carries them. A USB isolator can break a ground loop through the USB cable also.
If I read your diagram "shopbots" correctly, you are using flex conduit for part of the ground to shopbot 1. Run a real wire. Around 16 gage or bigger. The red connections in your diagram are a possible source of trouble if there is a neutral that is connected to that green connection shown coming from the box. Ground loops are a huge noise generator. (The most likely culprit).
Ground loops can usually be found with a clamp-on ammeter. Its common to find several AMPS of current flowing in circles in a ground loop. Plenty of current to cause a voltage shift adequate to interfere with an LVDS signal (the electrical method USB uses).
The ground loop issue would be made worse with different combinations of machinery running. My nickle is betting on that issue.
Hope that gives you a couple things to check, and probably remedy the trouble.
Keep us posted.
D
"The best thing about building something new is either you succeed or learn something. Its a win-win situation."
--Greg Westbrook