The resources of openstreetmap are severely limited, as the servers are donated, and on limited capacity connections.
See Tile Usage Policy for more information.
Some clients have been temporarily banned, in order to stop a particular client affecting the rest of the users. (see also Tile Abuse).
Bulk tile downloading features (e.g. for offline map browsing) tend to be in breach of the usage policy. Category:Tile downloading lists clients/apps doing this, however some such as OffMaps may be designed to avoid hitting OpenStreetMap servers (the recommended approach!)
e-OSM Client is a free client developed by e-Soft; you can connect e-OSM Client to any Open Street Map Tile Server; e-OSM Client has some limited batch downloading functions complying with the Tile Usage Policy and other interesting tools.
For more informations: E-OSM_Client
As outlined in this blog post on the navicomputer forums - announcing the release of version 0.93 - earlier versions were blocked, due to not complying with the Tile Usage Policy. Please upgrade to the latest version to continue to use Navicomputer with openstreetmap.