Hey Moggy,
I don’t know if it was the cause, but if on the original touch monitor, you connected the USB-C port that serves as power input by mistake to the Onefinity CNC controller (instead of connecting it to the wall power adapter), the monitor will be fed by the internal Raspberry Pi 3 instead, and this power suction could overload the Raspberry Pi.
It should be noted that the Raspberry Pi 3 inside the Onefinity Controller does not provide USB 3.0 ports (which provide higher current according to USB 3.0 specification), only USB 2.0 ports. By the USB 2.0 standard, in theory each of the CNC controller’s USB ports should be able to supply 100 mA in low power mode and a USB 2.0 device is allowed by the standard to draw up to 500 mA if it requests higher power modes from the USB host, however as a USB host, the Raspberry Pi 3 can only provide 1200 mA total current to USB peripherals.
Also the USB standard stipulates that devices initially start in low-power mode (100 mA or 150 mA) and, in the event of higher current requirements, first request this from the host before switching to normal mode. With USB 2.0 this can be up to four more times 100 mA, with USB 3.0 up to five more times 150 mA. If this request fails, the device must shut down. However, many devices only use the USB ports as a power source without asking, and violate the USB standard by drawing more than 100mA of current without permission from the host. In extreme cases, this could damage the USB port of the host or mess up the computer’s energy management, which can lead to unstable behavior.
– Source: Universal Serial Bus – Wikipedia (DE)
Note that if a Raspberry Pi gets too hot, it can become destabilized too. Since on the Onefinity CNC controller, the Raspberry Pi is enclosed together with the controller AVR/stepper drivers mainboard and a power supply with a 60 mm fan into a case which is ventilated only by a smaller fan, be sure to allow enough air to access both input and output of controller’s air flow and make sure you operate it inside the allowed temperature range of 4.4 °C – 43.3 °C.