Domain controller could not be contacted (Win2k3)

I have inherited a lingering issue, every 2 ? 3 weeks my client?s Windows Server 2003 looses its Domain controller, can not get in to the Active Directory, Exchange or anything that uses the Domain controller.

I have done a nslookup and the domain and IP address is resolved, I can connect to the DC manually in the Active directory, but if I close and re-open it , it doesn?t find the DC anymore.

Any advise would be greatly appreciated.