Loss of connection to SBS 2003

I have a SBS 2003 domain that I manage. I am frustrated because I keep having (xp pro) workstations dropp off. They show the network share mapped in my computer but can no longer access it. I can’t access it with a fully qualified path from command line either (i.e. \\server_name.domain_name.local\directory…). The compturs can no longer ping the domain name or server IP nor get email. I don’t see any red flags in AD and the only sure fix I have found is to remove and re-add to the computer to the domain. This is only temp. however. Please help!!