A few days ago I upgraded a virtual-machine host from Ubuntu 12.04 LTS (Precise Pangolin) to Ubuntu 14.04 LTS (Trusty Tahr). First, I thought that everything was working fine.
However, a short time later I noticed funny problems with the network connectivtity, particularly (but not only) affecting Windows guests. Occasionally, ICMP echo requests would only be answered with an enormous delay (seconds) or sometimes not even be answered at all. TCP connections to guests would stall very often. At the same time the load on the host system would be high even though the CPU usage would not be extremely heavy.
After I downgraded the virtual-machine host back to Ubuntu 12.04 LTS (and consequently to kernel 3.5) this problems disappeared immediately.
It seems like this is a bug related to the 3.13 kernel shipped with Ubuntu 14.04 LTS. There is a bug report on Launchpad and a discussion on Server Fault. It might be that the other problems that I experienced with the backported 3.13 kernel are related to this issue.
For the moment I will keep our virtual-machine hosts on Ubuntu 12.04 LTS and kernel 3.5, until the problems with the 3.13 kernel have been sorted out.