The server is a Windows Server 2003 Std w/ SP1 which is a domain controller,
fileserver and printserver.
During the last two days several DNS-errors have occured in ths DNS
Server-log:

Event ID: 4000
Source: DNS
Type: Error
Description: The DNS server was unable to open Active Directory. This DNS
server is configured to obtain and use information from the directory for
this zone and is unable to load the zone without it. Check that the Active
Directory is functioning properly and reload the zone. The event data is the
error code.

Event ID: 4004
Source DNS
Type Error
Description The DNS server was unable to complete directory service
enumeration of zone .. This DNS server is configured to use information
obtained from Active Directory for this zone and is unable to load the zone
without it. Check that the Active Directory is functioning properly and
repeat enumeration of the zone. The event data contains the error.

Event ID: 4015
Source DNS
Type Error
Description The DNS server has encountered a critical error from the Active
Directory. Check that the Active Directory is functioning properly. The event
data contains the error.

The 4000-event occurs most frequently.

What should I do to track this issue down?