intermittent drive mapping disconnects

I have a couple of machines on a Windows SBS 2003 network that, intermittently and during an active session, no longer connect to the mapped drives.  The mappings are visible in explorer but the folders are empty when you click on them.  The machines can still access the Internet, but all apparent traffic to the server stops.  If you reboot the machine, everything comes back up normally and work resumes.  It is a random event, random machines, random times.  I'd sure appreciate any thoughts on how to track this down.  Machines are XP professional, they've got active antivirus, have been scanned and are clean.  Thank you for any and all comments and suggestions.