W32Time Error Event ID 29, NtpClient has no source of accurate time

Below is the error and some of solutions i have tired but failed to work:

The Problem - W32Time Error, Event ID 29, Description="The time provider NtpClient is configured to acquire time from on or more time sources, however none of the sources are currently accessible.  No attempt to contact a source will be made for 1 minutes.  NtpClient has no source of accurate time."

The subsequent system log message says "The time provider NtpClient is currently receiving valid time data from DC"

It puzzles me why it would produce any error if it's still receiving valid time data.  I found the difference is that there's no message saying "time service is now synchronizing the system time with the time source DC."  So I have tried to find the answer online to this issue but cannot find one that matches my problem.  

The system is running Windows Sever 2003 Standard with Service Pack 2, it is a member server that syncronize its time with two domain servers.  Microsoft suggested to update the OS, but it already has the latest patches and SP2 (latest up to August 2008).  The configuration was set the same way as all the other servers, but other servers didn't show any problem.  The firewall was properly configured to accept NTP port 123 (on the DC side).  The registry on this server's W32Time was set exactly the same as other servers.  I even opened CMD and added the NTP servers again with the following commands:

NET TIME /QUERYSNTP
NET TIME /SETSNTP:
NET TIME /SETSNTP:"DC1 DC2"
restarted Windows Time service
w32tm /resync

If anyone who has knowledge what this problem is, then your input is greatly appreciated.  Any suggestions where to tackle this problem is welcomed.