I am working with some legacy PCBs containing USB-to-TTL UART adapters. The adapters are powered via the USB bus with only the transmit, receive and ground pins interconnected to the rest of the PCB. These adapters appear to Windows (7, 64-bit) as COM ports. Intermittently on powering up the system the COM port assignment of one of the boards will change. I'm trying to figure out what would cause that behavior. It seems the most likely culprit is a grounding issue since the port change mostly happens during power up and that's one of the very few electrical connections between the adapter and the rest of the board.
However, for some of these boards, moving the USB cable from one port on the PC to another will also cause a change in the COM port assignment regardless of whether or not the board is powered up.
Where should I be focusing my attention to track down this issue?
The cables are good, the drivers are up to date and the interconnects follow the manufacturers reference design.