Network connection lost

We have a strange issue under ONE 4.2.0: from time to time our VM mailer (Ubuntu 12.04 + Bluemind) loses the network connection.
ssh brutally cut, mail users receive a lot of network error messages etc.
the problem is random & lasts only one or two seconds.
Nothing special in the /var/log/syslog
The node which hosts the VM, the mailer server work perfectly, I changed also the network cables without success.
I have other VMs on this node which work fine as well (but under Debian).
So can you help me to solve this issue?
thanks in advance,