[LAU] Jack frequently crashes when running LinuxSampler + Qsynth

[Guru Prasad B. R.]

Hello everyone,
  This is my first post on this list. My compliments & gratitude to 
everyone working towards Linux Audio! I'm a semi-pro musician 
(keyboardist + singer), and for the past few months have been working on 
a linux-based setup for live performances. Here's what I use:
1. LinuxSampler, which loads my main piano sample. I depend on this 
sample throughout the performance (it happens to be a 888 MB sample). I 
also have additional pianos and drums loaded on. The other pianos are 
more of a backup, but I almost never use them. Drums are triggered by a 
Korg NanoPad.
2. Qsynth, on which I have a few string soundfonts on different 
channels, totalling less than 20 MB. Typically these are layered on top 
of tmy main piano sample.
3. I use a python script based on Mididings to for midi processing, to 
achieve splits and layers, and map hardware controllers.
4. JackMixDesk as a software mixer, to adjust the levels of the various 
layers, and vocals.
5. Jack-Rack, loading Calf reverb plugins + amplifier, for vocals.
6. Everything started through QJackCtl.

I use a Casio keyboard as a Midi controller (thru USB) and a Korg 
NanoKontrol. I recently migrated from a Celeron laptop to a core2 duo 
Macbook, both running Ubuntu 9.04. On both, I've been using the builtin 
soundcards. After migrations, there's been a persistant issue with 
Jack's stability:
-After a few minutes of playing on the keyboard, the sound from the 
samples become horribly distorted, crackly, buzzy and tinny (sorry, 
thats the best description I can give!).
-At the instant of this 'crash', this appears in the Messages window of 
qjackctl: "subgraph starting at LinuxSampler1 timed out 
(subgraph_wait_fd=31, status = 0, state = Finsihed, pollret = 0 revents 
= 0x0)
-When I restart Qsynth, it says "Qsynth1: Failed to create te audio 
driver (jack). Cannot continue without it".

Upon restarting Jack, everything's happily back to normal, until the 
crash happens again. I checked with the system monitor, and the resource 
usage is well within limits - max of ~80% CPU/core, and less than 50% of 
RAM being used at any point of time.
Needless to say, this effectively rules out using my linux system as a 
live performance tool. Like all stage musicians, I place a VERY high 
premium on stability. But I would love to have a completely open-source 
setup - so any pointers/help would be greatly appreciated!
Cheers,

Guru