Sporadic kernel "general protection fault"

[km at acrasis.net]

The PC is mostly used as a Samba server in a small office.  About once a
day this appears in syslog (with some snippage and reformatting):

  kernel: [160092.556290] general protection fault: 0000 [1] SMP
  kernel: [160092.586106] CPU 0
  kernel: [160092.598712] Modules linked in: binfmt_misc ppdev tun ipv6 [snip]
  kernel: [160093.327034] Pid: 28079, comm:
      smbd Not tainted 2.6.20-16-generic #2
  [snip]
  kernel: [160093.786906] Process smbd (pid: 28079,
      threadinfo ffff81000ba2a000, task ffff81001882e040)

The process (28079 in this one) is left in a 'D' state and cannot be
killed.  The users report that at the same time, a file they are using
on the share becomes 'stuck' while saving.

About the PC,

  cab1$ lsb_release -d
  Description:    Ubuntu 7.04
  cab1$ uname -a
  Linux cab1 2.6.20-16-generic #2 SMP Sun Sep 23 18:31:23
      UTC 2007 x86_64 GNU/Linux

Anyone else seen this?
-- 
KM