doesn't hibernate when battery is low

I have my laptop configured to automatically hibernate when low battery is detected.

Sometimes it does hibernate when it should.

Sometimes it hibernates too late, so that the power goes off while it is in the process of hibernating.

Sometimes it simply never hibernates. I get the warning four or five times that tells me that the computer "will hibernate very soon", but it does not. It doesn't even try, it just stays happily awake untill the battery suddenly dies instantly. The indicator battery icon does blink red and empty, so the low power condition is definitely detected. This is not a hardware problem (battery being old and not properly detecting low power with the necessary time margin): the proof of this is that the warning does pop up, a lot of times. But for some reason the system fails to initiate hibernation when it should.

This is the same machine where I experience issues 715850, 715855, 715862, 715865, but I was told to file a separate bug report for each separate issue and this is a clearly different issue from the point of view of the observed (mis)behavior.

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: linux-image-2.6.32-28-generic 2.6.32-28.55
Regression: Yes
Reproducible: Yes
ProcVersionSignature: Ubuntu 2.6.32-28.55-generic 2.6.32.27+drm33.12
Uname: Linux 2.6.32-28-generic i686
NonfreeKernelModules: nvidia
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.21.
Architecture: i386
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: teo 1579 F.... pulseaudio
CRDA: Error: [Errno 2] No such file or directory
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xfebf8000 irq 22'
   Mixer name : 'Realtek ALC660-VD'
   Components : 'HDA:10ec0660,10430000,00100001 HDA:10573055,10431316,00100700'
   Controls : 19
   Simple ctrls : 13
Date: Wed Feb 9 17:13:53 2011
HibernationDevice: RESUME=UUID=b1b3e5fe-38e2-44c9-9e48-4564cd9f4d66
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release i386 (20100429)
MachineType: ASUSTeK Computer Inc. M51Sn
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-28-generic root=UUID=173acbf5-26a0-49e3-8d2b-f1c142582cbf ro quiet splash
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.utf8
 SHELL=/bin/bash
RelatedPackageVersions: linux-firmware 1.34.3
SourcePackage: linux
dmi.bios.date: 12/24/2007
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 303
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: M51Sn
dmi.board.vendor: ASUSTeK Computer Inc.
dmi.board.version: 1.0
dmi.chassis.asset.tag: ATN12345678901234567
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK Computer Inc.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr303:bd12/24/2007:svnASUSTeKComputerInc.:pnM51Sn:pvr1.0:rvnASUSTeKComputerInc.:rnM51Sn:rvr1.0:cvnASUSTeKComputerInc.:ct10:cvr1.0:
dmi.product.name: M51Sn
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK Computer Inc.