That's a classic and frustrating issue. Since you've ruled out WiFi, focus on these two things:
Power: Your 12V/5A PSU might not be enough, especially if you're running a cooled camera and a mount. Voltage drops cause disconnects and reboots. Try a more powerful supply (e.g., 10A).
USB: A bad USB cable or a device causing a power surge on the USB bus will crash the connection. Disconnect all USB devices and reconnect them one by one with high-quality cables.
If those don't work, a Factory Reset (Settings -> System) is your next best step before considering an RMA.