[libvirt-users] Hypervisor loses network connectivity

I have been running KVM on a CentOS 7.1 host for a few months. I have one "main" host server plus a "backup" host server. Some time ago everything was running fine on "main", but all of a sudden the server became non-contactable on it's network interface. I could no longer SSH in and therefore couldn't run virt-manager. I don't know what caused this and I had to bring all the VMs up on "backup" and they've been running OK there ever since. Over the last few days I've rebuilt "main" and again all was going well and I migrated 4 of the 5 VMs back onto it. Then this morning I came to set up the final VM on "main" which is a Sophos firewall machine with 3 NICs (I have 3 physical NICs). Nothing uses 2 of the 3 NICs except the Sophos VM. I unplugged one of the ethernet cables from "backup" and plugged it into the 3rd NIC on "main" and all of a sudden "main" froze and again all network connectivity was lost. I rebooted it but the problem still remains that it's not contactable over the network and is therefore totally non-functional. When I log in at the console and run "ip a" I can see various network interfaces including br0 with the expected IP address - so it looks like it's got network connectivity but it hasn't. Has anyone else experienced this and know why it happens and how to fix it? I don't think I fancy rebuilding everything a 3rd time as it seems likely to happen again. Depending on why it's happening I'm thinking of moving off CentOS onto Ubuntu as the host OS, or moving away from KVM altogether. Thanks in advance for any help you can give.