Sebastien,

I have a CPU that scales from 3.00 GHz to 2.00 GHz.  This is the only power management I use.  That is the "ondemand" governor for my CPU and it was enabled by default.  I made no further changes to any power management settings in Ubuntu.  I get the issue even when pulseaudio is removed from my system and confirmed not running through "ps ax".

This might be worse with SMP although I'm not exactly sure why.

I have limited down the problem to a few packages.  When I install the Feisty packages listed here on my Hardy, the volume control functions properly (although the volume icon is missing, that is irrelevant).  When I install the Gutsy ones, it breaks.  Between Feisty and Gutsy is where the breakage happened.

From Feisty {
-rwxr-xr-x 1 andy andy 189352 2007-04-12 16:41 gnome-settings-daemon
-rw-r--r-- 1 andy andy  68400 2008-07-28 02:09 gstreamer0.10-alsa_0.10.12-0ubuntu1_amd64.deb
-rw-r--r-- 1 andy andy 480170 2008-07-28 02:09 gstreamer0.10-plugins-base_0.10.12-0ubuntu1_amd64.deb
-rw-r--r-- 1 andy andy  19876 2008-07-28 02:11 libgnomekbd1_2.18.1-0ubuntu1_amd64.deb
-rw-r--r-- 1 andy andy  20826 2008-07-28 02:11 libgnomekbd-common_2.18.1-0ubuntu1_all.deb
-rw-r--r-- 1 andy andy 687866 2008-07-28 02:09 libgstreamer0.10-0_0.10.12-0ubuntu2_amd64.deb
-rw-r--r-- 1 andy andy 181430 2008-07-28 02:09 libgstreamer-plugins-base0.10-0_0.10.12-0ubuntu1_amd64.deb
-rw-r--r-- 1 andy andy  80516 2008-07-28 02:09 libxklavier11_3.2-0ubuntu1_amd64.deb
}

- problem comes in here -

Gutsy {
-rwxr-xr-x 1 andy andy 209704 2007-10-08 06:48 gnome-settings-daemon
-rw-r--r-- 1 andy andy  71246 2008-07-28 02:07 gstreamer0.10-alsa_0.10.14-1ubuntu3_amd64.deb
-rw-r--r-- 1 andy andy 528782 2008-07-28 02:12 gstreamer0.10-plugins-base_0.10.14-1ubuntu3_amd64.deb
-rw-r--r-- 1 andy andy 738756 2008-07-28 02:08 libgstreamer0.10-0_0.10.14-1ubuntu3_amd64.deb
-rw-r--r-- 1 andy andy 231534 2008-07-28 02:08 libgstreamer-plugins-base0.10-0_0.10.14-1ubuntu3_amd64.deb
}

Furthermore,

I modified the Gutsy control-center-2.18.1/gnome-settings-daemon/actions/acme-volume-gstreamer.c and control-center-2.18.1/gnome-settings-daemon/gnome-settings-multimedia-keys.c files to match Feisty's control center package.

Then I modified the gutsy source package gst-plugins-base0.10-0.10.14's ext/alsa directory to the Feisty gst-plugins-base0.10-0.10.12 ext/alsa.  After compiling and loading the new Gutsy debs onto my Hardy, the volume control functions properly.  

sudo dpkg -i gstreamer0.10-alsa_0.10.14-1ubuntu3_amd64.deb 

sudo dpkg -i gstreamer0.10-plugins-base_0.10.14-1ubuntu3_amd64.deb 

sudo dpkg -i libgstreamer-plugins-base0.10-0_0.10.14-1ubuntu3_amd64.deb 

So I loaded in my patches.  I copied the gnome-settings-daemon out of the new deb, placed in Hardy's /usr/bin.  Killed the process and restarted it.  Then using the keyboard, the volume keys work properly as I can see by running alsamixer in the background.  No jumping to zero or mess ups. gnome-volume-control also acts much more reliably with the patched Gutsy libs.

Certainly I will be working on a patch to Hardy and Intrepid for this but there was a lot of code that was changed (few hundred lines?)  So it may take some further investigation or intervention by the actual developers on what went wrong between gstreamer rev 10.12 and 10.14.  But, the problem is now isolated to the gst-plugin-base ext/alsa directory between 10.12 and 10.14 (and possibly gnome-settings-daemon but that's more of a front-end change).  I hope this helps.

I am going to try and limit down the problem further and provide a patch but may need assistance there.