Tried an experiment.
Connected the FT100 + sound card interface + USB to serial interface, etc. to a computer running Windows 7 - problem disappears!
Then connected it to a laptop running Windows 8.1 - problem re-appears - keys PTT every 30 seconds again.
This appears to be a Windows 8 issue,
but will have to see whether changing interface or USB-serial adapter solves it.
This only proves there is a issue with driver your using with Win 8. I bad driver does not make a OS bad. With each revision of Windows MS makes code requirements more rigid meaning some sloppy code that might run on 7, will not run on 8. For many years MS was slow to raise standard and XP was very tolerant. Vista was a big step in compliance and 8 another. Big steps can cause software/driver issues for a while.
True, but I've learned a lesson - wait until everyone has sorted the bugs out before upgrading.
The driver I am using is by Prolific - though I guess the design requirements didn't include keying a transmitter.
I would like to know why the USB port is being polled every 30 seconds.
Whether its a driver issue or just part of the USB interface specification (though from a quick look at this I can't see it).