Thanks for letting me know. I'm running at 24 bit 48Khz now, but no difference in the lockups. I have noticed that the lockups seem to *start* when I do something that might cause a bit of processor / disk IO e.g. change to a different program, maximize a window, etc, then they carry on for an unspecified time until normal service is resumed.
Perhaps this could be an interrupt issue? Although it (from memory) only happens when jack(dbus) is also running, so maybe it's an interaction. I've just stopped jack and restarted pulseaudio (when Jack stopped, my Pulseaudio was full of artefacts - that's new), and I'll see how it goes for the rest of the evening.
In any event, I've just ordered a new interface with SPDIF (Behringer UMC1820), but it would still be great to get to the bottom of this issue with the Lambda. Perhaps it's not hardware-specific, I guess I'll know for sure when the new interface comes, if I don't fix the issue by then.