I think we might have found a solution to this...
After doing some research on the web, I found that people had had problems using cables that did not have the FTDI chipset. The other type of chipset in cheaper cables is called Prolific. I believe the first cable I bought was a Prolific chipset type. When I bought a cable with a FTDI chipset the connection problem seemed to disappear.
I assume that the cable you sell has a FTDI chipset. If anyone else contacts you with connection problems, ask them to check that they are using a cable with a FTDI chipset.