I managed to solve a bit more of the lockup puzzle last night... looks like
missing libraries.
I'm running Debian unstable; and it seems they package the Mesa libraries in
a couple of packages, and don't tell you which ones you need. :( 
after I added the xlibosmesa3 package ('offscreen rendering for mesa'), I
can now run for about 1/2 hour before the screen freezes. (instead of 1-2
minutes).

here's the mesa stuff I have installed:
chrome at steel:~$ dpkg -l *mesa*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err:
uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
un  mesa-dev       <none>         (no description available)
ii  mesademos      3.4.2-1        Example programs for Mesa (and OpenGL in
gen
pn  mesag-dev      <none>         (no description available)
pn  mesag-glide2-d <none>         (no description available)
pn  mesag-widgets- <none>         (no description available)
pn  mesag3         <none>         (no description available)
pn  mesag3+ggi     <none>         (no description available)
pn  mesag3+ggi-dev <none>         (no description available)
un  mesag3-glide   <none>         (no description available)
pn  mesag3-glide2  <none>         (no description available)
pn  mesag3-widgets <none>         (no description available)
pn  xlibmesa-dev   <none>         (no description available)
ii  xlibmesa3      4.1.0-9        XFree86 version of Mesa 3D graphics
library
pn  xlibosmesa-dev <none>         (no description available)
ii  xlibosmesa3    4.1.0-9        Mesa/XFree86 offscreen rendering library

Any idea what I might still be missing?

Carl Soderstrom
-- 
Network Engineer
Real-Time Enterprises
(952) 943-8700