General :: SSH: Server Unexpected Closed Network Connection

In my case, it occurs intermittently when trying to connect using putty's command line client 'plink.exe' to a linux machine using ssh. For some testing purposes, a linux server is setup with 8K virtual ip addresses and the testing scripts will connect to these ip addresses through ssh using plink.exe. There will be concurrently 25 ssh sessions launched to the server. While it works for sometimes, it throws 'server unexpectedly closed network connection' error intermittently. If the same ip address is tried again manually, it works. While the ssh server is accepting connections, this error is thrown sometimes and unable to find out the reason from the logs. It is not that the ssh connection is completely blocked. It gives this error message while connecting to some of the virtual ip addresses but works some times.

The hosts.deny, sshd_config are tuned to allow the connections, allow more sessions etc.