Sorry, I can't quote easily (using Gmail on the web)...
About underruns, I'm using 128 frames / period and 2 periods / buffer. Now I'm trying to reproduce the issue without jackd, I guess that might be less relevant. The "operations" I was talking about can hardly be called massive, and the lockups last for much longer than the operations themselves.
it's a pretty decent i5, nothing earth-shattering, but then I'm not asking much of it. The lockups happen even without Ardour, just listening to music with Audacious / VLC / Chrome (YouTube) / listening to old demos on Audacity.
antony@cubase:~$ sudo hdparm -t /dev/sda
/dev/sda:
Timing buffered disk reads: 532 MB in 3.00 seconds = 177.30 MB/sec
antony@cubase:~$ sudo hdparm -T /dev/sda
/dev/sda:
Timing cached reads: 23756 MB in 2.00 seconds = 11889.97 MB/sec
At the moment, the jury's still out on whether the lockups happen without jackd. Currently running without it and will report back either way.