This wiki article collects the possible reasons of this happening: (note, we collected issues here when either the BIM server stops, or the client goes offline while the BIM server keeps running)
http://archicadwiki.com/Bugs/Teamwork/BimServerStopsRegularly
Also make sure that you are on the latest build (3600 in case of AC 13), since some of these problems may have been fixed since the article was written.
If none of the suggestions help, contact your local support and send the logs listed in the article.
HTH,
Greg
Gergely Kmethy
VP, Customer Success, Graphisoft