server problems

Hi all

we are having a few problems with one of our active directory servers

We keep getting errors in the event viewer every  15 minutes in the directory service
the logs are

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Event Type:      Error
Event Source:      NTDS Replication
Event Category:      DS RPC Client
Event ID:      2087
Date:            17/06/2008
Time:            9:43:15 AM
User:            NT AUTHORITY\ANONYMOUS LOGON
Computer:      VTAD01
Description:
Active Directory could not resolve the following DNS host name of the source domain controller to an IP address. This error prevents additions, deletions and changes in Active Directory from replicating between one or more domain controllers in the forest. Security groups, group policy, users and computers and their passwords will be inconsistent between domain controllers until this error is resolved, potentially affecting logon authentication and access to network resources.
 
Source domain controller:
 segad01
Failing DNS host name:
 a5b574aa-0b30-467e-8388-3fba7c74b9d7._msdcs.melbourne.shock.com.au
 
NOTE: By default, only up to 10 DNS failures are shown for any given 12 hour period, even if more than 10 failures occur.  To log all individual failure events, set the following diagnostics registry value to 1:
 
Registry Path:
HKLM\System\CurrentControlSet\Services\NTDS\Diagnostics\22 DS RPC Client
 
User Action:
 
 1) If the source domain controller is no longer functioning or its operating system has been reinstalled with a different computer name or NTDSDSA object GUID, remove the source domain controller's metadata with ntdsutil.exe, using the steps outlined in MSKB article 216498.
 
 2) Confirm that the source domain controller is running Active directory and is accessible on the network by typing "net view \\<source DC name>" or "ping <source DC name>".
 
 3) Verify that the source domain controller is using a valid DNS server for DNS services, and that the source domain controller's host record and CNAME record are correctly registered, using the DNS Enhanced version of DCDIAG.EXE available on http://www.microsoft.com/dns 
 
  dcdiag /test:dns
 
 4) Verify that that this destination domain controller is using a valid DNS server for DNS services, by running the DNS Enhanced version of DCDIAG.EXE command on the console of the destination domain controller, as follows:
 
  dcdiag /test:dns
 
 5) For further analysis of DNS error failures see KB 824449:
   http://support.microsoft.com/?kbid=824449
 
Additional Data
Error value:
 11004 The requested name is valid, but no data of the requested type was found.


For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
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and
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All domain controllers in the following site that can replicate the directory partition over this transport are currently unavailable.
 
Site:
CN=Melbourne,CN=Sites,CN=Configuration,DC=melbourne,DC=shock,DC=com,DC=au
Directory partition:
CN=Configuration,DC=melbourne,DC=shock,DC=com,DC=au
Transport:
CN=IP,CN=Inter-Site Transports,CN=Sites,CN=Configuration,DC=melbourne,DC=shock,DC=com,DC=au

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

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any thought why we are getting this error messages? and how to stop it