[other] libvirt issue on ubuntu 12.04

Hi

My system: Ubuntu 12.04.1 , kerenl = 3.2.0-25-generic . libvirt and virsh version: 0.9.8
where libvirt came from the ubuntu standard install.

PROBLEM: I get from time to time the following error message :

"libvirtError: Failed to connect socket to '/var/run/libvirt/libvirt-sock': No such file or directory"

this happens after : Connecting to libvirt: qemu:///system from (pid=1363)

The libvirt-sock is a local domain socket (ls -l gives:
srwxrwx--- 1 root libvirtd 0 Sep 3 13:56 /var/run/libvirt/libvirt-sock )

NOTE the libvirtd is up before and after the above error message as shown by 'ps' output :
root 1574 1 0 16:23 ? 00:00:00 /usr/sbin/libvirtd -d -l


On a similar system with a slightly prior kernal version: 3.2.0-20-generic , there is no such problem . ANY IDEA WHAT TO DO ?