2003 server drops connections

I have a Windows 2003 server running SQL server.
there are a few pcs on my network that randomly loose their connection to this server. the sql application will hang on startup or just freeze when the user is inputiing data. If I then ping the server from their pc it times out, but I can ping all the other servers on the network from the same pc. the majority of pcs work fine to this server. I would really appreciate some help and advice on this as I have run out of ideas
Thanks in advance
Steve