Lots of Flushing x bytes to y would block messages

[Terry T]

We run tinc in a linux environment in which it sits there waiting for
connections from the clients. All clients are configured to only have one
ConnectTo which points to this server.

We're seeing in the server log that as soon as a client's connection is
activated, a whole bunch of "Flushing x bytes to that host would block" is
logged and the whole vpn is bogged down and has become non-responsive.

>From a Jun 27, 2013 "Metadata socket read error" thread, Gus suggested that
it may be caused by the router in front of the server that ran out of
memory keeping track of the outgoing connections. The server is indeed
behind a NAT router. It's a reasonably (ASUS) high end consumer grade
router.

We have tried adding PingInterval in the host configuration file, but it
doesn't seem to resolve the problem. Is there anything else that we can do
to stop the vpn from crashing. As it is, when any one of the client is
logged to be flushing, the whole vpn becomes unusable.
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