During the latest changes for the Home Server articles I replaced the VirtualBox network drivers with the kernel’s virtio drivers. This was around the same time I was experimenting with traffic shaping and some other things, so when network problems would show up it was not right away clear what caused it. All the other computers had no problems while they were several virtio drivers in the path.
What exactly caused the troubles is a bit of a mystery, but it seems to work well again.