Thanks, I'm not new to any of this stuff. My tests aren't "random", nor are they "tests" as such, I'm seeing issues during my normal daily usage. I don't *only* use Ardour, it's a multi-purpose workstation, and PulseAudio gives me a lot of useful flexibility which I'm not going to give up unless it's intrinsically too buggy to use. By and large, the pulseaudio-jackdbus integration is great.
In any event, even after jack is disabled, the lockups seem to have continued. I say "seem to", because I'm testing old audio files, some of which are corrupt from an old bad hard disk, so I need to make sure that any strange behaviour is the same as I've seen earlier.
I also don't remember such lockups with pulseaudio and jackdbus and my old interface (the supposedly crappy Behringer UCA202 which never caused me any trouble), although both hardware and software have changed since then, so who knows...