Thanks for your thoughts guys.
Clock mismatch was the clue :o
I had been a bit lazy. While switching the output device I'd neglected to also
switch the input device, so jack was seeing two slightly different 48kHz
clocks. Switch both at the same time and the problem disappears.
As a matter of curiosity I wonder if we can work out what the actual difference
is. Would the Xrun occur when there was a one frame difference, one period, or
1 buffer? It's running at 256 frames per period and 2 periods per buffer. The
xruns occur every 11 minutes (and a few seconds).
As I said at the start, this is my 'office' machine and has *no* optimisations.
--
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.