I recently upgraded to Ubuntu 13.04 and that is when this issue started for me.
When the problem is occurring, local packet captures show traffic attempting to be transmitted but there isn't any remote traffic present, even broadcast traffic on the segment no longer appears.
The problem starts once a vm guest is running and transmits traffic for a short period.. no more than 20 seconds. All host and guest connectivity is affected.
You can jump start the interface briefly if you physically remove the ethernet cable and plug it back in.
I'm running the guests with NAT based connections so there isn't an overriding ARP broadcast or something weird like that.
If I disconnect the virtual nic on the guest and replug the ethernet cable, the host OS will have connectivity.
This only happens with the ethernet interface. If I run off the wireless interface on the host then everything is fine.
Anyone have any thoughts about this?