I have seen this as well, while I was using the USBtoSerial adapter. As my system had a USB-UIRT on it, on a reboot, sometimes the device ID would change and /dev/ttyUSB0 would become /dev/ttyUSB1. I have since then purchased a serial daughter card for the header on my mobo, and now the issue has gone away. I thought it was just dumb luck, but now as you have experienced the same thing, I guess we can chalk it up to the USB bus.
My system is currently functioning with a CM11A, 19 X10 devices, and 3 Insteon Devices.