[ warning] [vmusr:vmusr] Error in the RPC receive loop: RpcIn: Unable to send

Hi,
We have a new server installation.
We running a lot of servers  Windows 2008r2 in the Esxi 5.1 Vmware, some times the users say me that  the RDP session it freeze for 10 seconds aprox. User connect with Cisco  Ipsec Client for windows.
We found in the Aplication Event Viewer log this message:
"[ warning] [vmusr:vmusr] Error in the RPC receive loop: RpcIn: Unable to send."

And in the System Event Viewer log this message with the same time:
"The  Terminal Server security layer detected an error in the protocol stream  and has disconnected the client. Client IP: 192.168.xxx.xx."

Any suggestion?
Thanks.