It seems strange that the PMDX-416 works OK if it is found but Mach4 has problems finding the PMDX-416 when starting. This could be a software problem related to the configuration on your computer or Mach4.
With Mach4 not running, try unplugging and replugging the PMDX-416 several times. Give it a couple of seconds for Windows to react. You should get a sound or other notifications from Windows each time you plug or unplug the PMDX-416 just like those you get when plugging and unplugging a mouse. You can use this as a test case (not involving Mach4) for trying it on different ports, different cables, or even different computers.
Next, test the PMDX-416 with only the USB cable connected. You can easily disconnect the PMDX-416 from all other wiring by unplugging the terminal strips without unscrewing your wiring harness connections.
If the PMDX-416 fails to connect reliably in these isolated tests, please return it for warranty repair.
Sometimes with USB there can be problems with a "ground loop" between the computer ground and the machine ground. If you find that the PMDX-416 can be made to work if the only thing wired to it is the USB cable, and all other terminal blocks unplugged, then suspect a ground loop. If you do see this situation there are two things that you can try. First make sure that the computer's power and the safety ground at the machine are coming from the same power outlet. Second, you can add a heavy wire (like 18 gauge lamp cord) between the metal chassis of the computer case and the "GND" terminal of J7 on the PMDX-416. This will shunt the "ground loop" leakage current through the heavy wire instead of through the shield wire of the USB cable.
I do not have any experience with trying an Ethernet to USB adapter, but such configurations do involve extra layers of software that can make configuration more complicated.
Regards,
Steve Stallings
PMDX