LVM logical volumes get deactivated on reboot

[Till Wimmer <g4-lisz@tonarchiv.ch>]

Hi,

i set up a Debian system with the root filesystem located on a LVM 
logical volume:

/dev/sda1 -> /boot (with GRUB)
/dev/sda2 -> swap
/dev/sda3 -> LVM physical volume -> volume group "system"

The volume group "system" contains "root".
The kernel line in GRUB's menu.lst looks like this:
kernel          /vmlinuz-2.6.26-2-amd64 root=/dev/mapper/system-root ro

From time to time booting of this machine fails: Device 
"/dev/mapper/system-root" could not be found.
All volumes in the "system" group are deactivated for any reason.

Reactivating them from the rescue busybox system works without any problems:
vgchange -a y

What could be the reason for this issue?

Thank you.
TW