On Sun, 2005-11-06 at 16:48 +0100, federico wrote:
  it happens randomly, while I play a soft synth, that
jack XRUNs once,
 and the audio become crappy, noisy, like a kinda of digital
 effect....bit crusher? 
It's probably a driver bug but could also be a PCI latency issue.
Try setpci -d CARD:ID latency_timer=40 where CARD:ID is the PCI id of
the card obtained from lspci.  Do this immediately after loading the
driver and before playing any audio.
Lee