Hi Paul,
thanks for your comments.
The vin (vout) is not used as a supply it is connected to the out pin on a LM7806 though.
In stand-alone mode the Arduino is powered from a 12V supply which is reduced to 6V by a LM7806. The power supply circuit is based on Derek Molloy’s video from Youtube. In stand-alone mode the Arduino works fine.
Before adding the power supply circuit the Arduino was always powered from the USB and I never had an issues with the host computer. It started crashing after adding the circuit which is why I suspected the vin pin. The vin pin was not used prior to adding the new circuit.
Not clear from the above but the crash happened when I unplugged from the USB. While plugged in it seemed to work OK.