It happens 2-3 times a day, a long period of time will happen once. This must be really hard to find source of a problem. As Balazs said, he would use an oscilloscope to monitor at dc power supply with trigger stop. I think this is a best tool is used to track down the problem.
You might use a cheap loop recording camera shooting at ordinary multi- meter, on board LEDs, latency graph of UCCNC or any things that could shows status and events. Then come back to see it right before it stopping. You can stop camera by using CP signal cutting down the power from the camera. This signal will stop if UCCNC communication disconnect from UC300ETH-UB1.
Or, you may start from changing relevant devices, such as.
-Power supply, Cables, Computer etc.
-The DC2DC can be disconnected by removing solder-bridge then connect external 5Vdc to GND and VCC terminal to power UC300eth instead. Please consult the manual.
-UB1 or UC300ETH board. This case, I can offer you 50% discount for the second UB1 board, but please send me the photo of your UB1 because I couldn't find your information in my online store records.
The isolated transformer and EMI filter are preferred to protect and reduce noise in industrial environment.
Please measure spindle mounting plate, cabinet ground against SECOND earth ground with volt meter again. High voltage leaking could disrupt controller. The old ground rod could become rotten and broken. If there is an electricity leak on those metal plate and the ground rod is not so good, we can feel it by touching them.
Thanks
Weerasak