I can ping our File server from clients but, copying fails every few minute

We have a Windows Server 2003 sp2 setup as a file server which is connected to a Dell EqualLogic Disk array. there are lots of file transfers, and file sizes are from 4 GB to 30 GB.
the problem that we are facing is that, when users try to copy files it disconnects with a message (cannot copy X because the network name is no longer available). but at the same time i get ping replies and file transfer through FTP works fine. we can connect to share again by it’s UNC Path but the folders opened on the share closes and copying disconnects.
Please help me!