I wonder if your router isn't passing through the messages after a time out. (again, I forget the specifics) for the problem (maybe not this one) the workaround was sending a packet periodically to indicate to the router that you still want the communications to continue on that port.
I wonder if your router isn't passing through the messages after a time out. (again, I forget the specifics) for the problem (maybe not this one) the workaround was sending a packet periodically to indicate to the router that you still want the communications to continue on that port.