Unexplainable Disconnect From A Domain

I have quite few Windows XP PC’s that will seem to disconnect from a domain and the Local Administrators group will not show the “Domainname”\Domain Admins instead it shows a long Alpha-Numeric string. This is happening on different domains. The only way to fix this problem that I have found is to drop it down to a Workgroup and then rejoin it to the domain. Why are they being disconnected in the first place? Any ideas???