Intermittent server connection dropouts and/or failures upon (re)connection.
Hardwired connections - Turn Off Airport.
If a server timeout occurs, Airport may obtain an IP address, before attempting reconnection.
If possible manually input a static IP address toward the higher range of the IP pool.
Creating a new location may help if the network is used frequently.
'Network' preference pane, hold down 'Location:', select 'Edit locations' [or 'New location'] from the pop-up menu, click the plus sign [or select a name] to add a new location.
Wireless connections - Check your Airport network.
If a server timeout occurs, or the signal drops, Airport may switch to another network.
In the 'Network' preference pane, check the 'Ask to join new networks' option.
You may need to delete previously joined open networks from the 'Advanced' - 'Preferred Networks' list.