Hi. I've seen this on two systems now. Both could be classified as live network streaming gateway devices (a Citrix NetScaler gateway, and just now a remote support tool that has quite a few clients connected to it).
When the ghettoVCb backup to an NFS datastore is taking place, the streaming data connections grind to a halt.
Could it be that the management interface (ESXi shell) has a higher priority for networking than the guests?
The guests and management interface are all one/shared.
Do you think dedicating a NIC that isn't the management-NIC to the guests (actually there's only one guest on this box anyway now) would solve the issue?