Geoff Beasley wrote:
  You are not running a RT kernel; that's your
problem. For any serious
 audio work you MUST run a RT kernel. You can roll your own quite easily
 if you wish, or use one of a number of specially prepared audiocentric
 distro's for linux; eg: Planet CCRMA, 64 Studio,Musix and many others. 
Aha! It turns out that there is no official Arch RT kernel (for whatever
reason). However, some nice person has a feed with an RT patched kernel
for Arch- I've installed that and now I have no xruns- woohoo!
Thanks
Simon