Just out of curiosity, are these non-working devies FTDI devices (and have you used D2XX drivers) or PL2303 or something else? It would be interesting to pinpoint what actually causes this, poor quality USB-serial bridge, fake and poor quality ELM327 clone or what...
Mine was a PL2303 clone for £5 off Ebay.
Fine for doing all the Ford mods I want except PCM flashing.
Main problem I've had is finding a Windows 7 serial to USB driver for it.
There isn't one, but oddly there is a Vista driver.
So I use my Vista lappy with it.