It tunrs out that this is down to a known bug with Microsoft Server 2008 R2.
Kernel sockets leak on a multi-processor computer running Windows Server 2008 R2.
WSAENOBUFS (10055) An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full.
Specifically, the application may be unable to create new sockets and a "no buffer space available" or "insufficient buffer space" exception occurs on the computer. Additionally, when this issue occurs users cannot remotely connect to the computer until it is restarted.
This issue occurs because of a race condition in the Ancillary Function Driver for WinSock (Afd.sys) that causes sockets to be leaked. With time, the issue that is described in the "Symptoms" section occurs if all available socket resources are exhausted.