If it's not covered by your references, make sure that the PC you use to flash the firmware has the power options set to keep the port alive no matter what.
The PC should not idle a port that is use, but it has happened to me so I now have a practice to always set my power options to the most environmentally unfriendly possible choices. I like to use a laptop that is plugged into a power outlet. If there is a power failure, the batteries keep her purring.
If you flash over Ethernet, use an isolated network, ideally just your PC and the PAC and a cable.