I have a Dell PowerEdge 2950 server running Hyper V. I have 6 guests on this host.
The domain controller for this network (Server 2003) is a VM on this host. Another VM is used for the mail server (Exchange 2003/Server 2003), as well as a file server on Server 2008.
If large files (over 10mb or so) are copied over our site to site VPN, they always fail with the message "specified network name is no longer available" however the mapped drive is still visible.
I did some testing, and this occurs when the file shares are on one of the VM's. If the files are directly on the Hyper V host itself, the issue does not occur. I can only assume its a network setting within the Virtual Network Manager or the VM's themselves...