Triggered a reboot:
Nov 22 11:34:38 Philinux kernel: [  106.085237] __ratelimit: 3 callbacks suppressed
Nov 22 11:34:38 Philinux kernel: [  106.085250] python[1917]: segfault at 900c0c ip 0808e110 sp bf8c0530 error 4 in python2.6[8048000+1d3000]
Nov 22 11:38:10 Philinux kernel: imklog 4.2.0, log source = /var/run/rsyslog/kmsg started.

Triggered a reboot:
Nov 22 11:39:35 Philinux kernel: [  113.942371] __ratelimit: 3 callbacks suppressed
Nov 22 11:39:35 Philinux kernel: [  113.942385] deskbar-applet[2308]: segfault at c35d5f9e ip 00d1ade7 sp bfafe724 error 5 in libc-2.10.1.so[c14000+13e000]
Nov 22 11:40:03 Philinux kernel: [  142.089533] BUG: Bad page map in process apport-checkrep  pte:00c92025 pmd:32b91067
Nov 22 11:40:03 Philinux kernel: [  142.089549] page:c104b240 flags:00000004 count:0 mapcount:-1 mapping:(null) index:0
Nov 22 11:40:03 Philinux kernel: [  142.089559] addr:002e0000 vm_flags:08000075 anon_vma:(null) mapping:f6db833c index:df
Nov 22 11:40:03 Philinux kernel: [  142.089574] vma->vm_ops->fault: filemap_fault+0x0/0x340
Nov 22 11:40:03 Philinux kernel: [  142.089582] vma->vm_file->f_op->mmap: generic_file_mmap+0x0/0x50
Nov 22 11:40:03 Philinux kernel: [  142.089591] Pid: 2988, comm: apport-checkrep Tainted: P           2.6.31-15-generic #50-Ubuntu
Nov 22 11:40:03 Philinux kernel: [  142.089598] Call Trace:
Nov 22 11:40:03 Philinux kernel: [  142.089613]  [<c01c8a89>] print_bad_pte+0x139/0x1f0
Nov 22 11:40:03 Philinux kernel: [  142.089622]  [<c01ca5e7>] zap_pte_range+0x3d7/0x3e0
Nov 22 11:40:03 Philinux kernel: [  142.089631]  [<c01ca866>] unmap_vmas+0x186/0x350
Nov 22 11:40:03 Philinux kernel: [  142.089640]  [<c01cf780>] exit_mmap+0xa0/0x160
Nov 22 11:40:03 Philinux kernel: [  142.089651]  [<c014361e>] mmput+0x2e/0xa0
Nov 22 11:40:03 Philinux kernel: [  142.089659]  [<c0147bb2>] exit_mm+0xe2/0x120
Nov 22 11:40:03 Philinux kernel: [  142.089667]  [<c0147ddc>] do_exit+0xfc/0x2e0
Nov 22 11:40:03 Philinux kernel: [  142.089676]  [<c01527dd>] ? dequeue_signal+0x2d/0x170
Nov 22 11:40:03 Philinux kernel: [  142.089685]  [<c0161dab>] ? sched_clock_cpu+0xeb/0x1a0
Nov 22 11:40:03 Philinux kernel: [  142.089693]  [<c0147ffa>] do_group_exit+0x3a/0xb0
Nov 22 11:40:03 Philinux kernel: [  142.089702]  [<c015403f>] get_signal_to_deliver+0x18f/0x300
Nov 22 11:40:03 Philinux kernel: [  142.089712]  [<c03b27b8>] ? scsi_finish_command+0x98/0x100
Nov 22 11:40:03 Philinux kernel: [  142.089721]  [<c010304b>] do_signal+0x6b/0x160
Nov 22 11:40:03 Philinux kernel: [  142.089730]  [<c0152f88>] ? send_signal+0x38/0x70
Nov 22 11:40:03 Philinux kernel: [  142.089738]  [<c0154227>] ? force_sig_info+0x77/0xc0
Nov 22 11:40:03 Philinux kernel: [  142.089747]  [<c015427f>] ? force_sig+0xf/0x20
Nov 22 11:40:03 Philinux kernel: [  142.089756]  [<c0571479>] ? do_trap+0xb9/0xc0
Nov 22 11:40:03 Philinux kernel: [  142.089764]  [<c010318d>] do_notify_resume+0x4d/0x60
Nov 22 11:40:03 Philinux kernel: [  142.089772]  [<c0103448>] work_notifysig+0x13/0x1b
Nov 22 11:41:52 Philinux kernel: imklog 4.2.0, log source = /var/run/rsyslog/kmsg started.

Triggered a reboot:
Nov 22 11:42:28 Philinux pulseaudio[1842]: ratelimit.c: 1 events suppressed
Nov 22 11:42:34 Philinux pulseaudio[1842]: ratelimit.c: 2 events suppressed
Nov 22 11:42:57 Philinux kernel: [   93.236038] Clocksource tsc unstable (delta = -165929002 ns)
Nov 22 11:45:22 Philinux kernel: [  238.053692] __ratelimit: 3 callbacks suppressed
Nov 22 11:45:22 Philinux kernel: [  238.053705] update-python-m[2997]: segfault at 97ba81c ip 0808e8db sp bfad1100 error 4 in python2.6[8048000+1d3000]

In all three cases, the crash followed immediately a 
 __ratelimit: 3 callbacks suppressed
message.

Even when the machine does not hard crash / freeze, I can see at least one desktop application crash (usually a segfault) for each reboot, during the time the desktop initializes. The crashing application varies from time to time. 
I have also seen several cases of a full freeze (or even a direct reboot) during the loading firefox.
And one time, I had a full freeze (keyboard and soft power button dead) when the machine speakers produced a loud, random "static noise": A noise similar to that of a radio tuned to no station on a short AM waveband.

It looks more and more to me that the problem is related to pulseaudio, but the next step would be to see if the problem still occurs with pulseaudio out of the way.
Could anyone tell me how to prevent pulseaudio and all of its drivers to load at all at boot time?