ssh packet_write_wait: Connection to Broken pipe

Hi all

I've recently upgraded my 2012 mac mini to El Capitan. I mostly use this machine to connect to my linux vms running in the basement of our building. I've always connected to these machines using the following ssh command:

ssh -Y -o TCPKeepAlive=no -o ServerAliveInterval=15 username@hostname.

These connections were rock solid before the upgrade. Now if I leave them up for several hours unattended the connection dies with the following error:

packet_write_wait: Connection to xxx.xx.x.xx: Broken pipe

where the x represents the ip address, not literal x characters.

Does anyone know why this is happening and how to stop it?

Thanks

Howard