Wild guess, sounds like the slave is forgetting how the host is with each power cycle, or vice versa. Maybe the settings are just not saving right. what kind of upgrade did you do on the OS's? Start from scratch, or did you upgrade from a previous OS? May want to start all fresh on all the machines in the network.
Are you using a proxy of any kind?

My suggestion would be just use a router, and not a server set up like that. A router is a bit expensive, but it saves a lot of hassales.