Hi Jim,
You can try removing the usb from the back of the HMI and try running from a laptop/pc (Downloads - RealTime CNC) (you’ll need to power on the HMI for power to the motors in the test), and see if you disconnects continue.
You case does sound like EMI for the screen.
(here’s the faq: EMI | Screen Flickering, Blank, or Black Screen, Randomly Loosing Zeros, Freezing, Disconnecting or Random Happenings)