SysChat - Network connection lost after logging into domain

Help! I am running a network with windows 2000 and 2003 servers. What problems I am having is that people will log on to the network and sometimes log in without problem. Other times they will not get connectivity (I think its with the DNS) and will have to reboot in order to get their computer to work. Other times once people connect, after some time they will lose their internet, and ability to connect to anything else. I usually have to repair their network connection, but that only last temporarily. I have attached below some of the errors from the log on both the client and server side:

client:
The kerberos subsystem encountered a PAC verification failure. This indicates that the PAC from the client EVENSTART03$ in realm ______.ORG had a PAC which failed to verify or was modified. Contact your system administrator.

No Domain Controller is available for domain _______ due to the following:
There are currently no logon servers available to service the logon request. .
Make sure that the computer is connected to the network and try again. If the problem persists, please contact your domain administrator.

The IP address lease 192.168.2.159 for the Network Card with network address 00111114A599 has been denied by the DHCP server 192.168.168.1 (The DHCP Server sent a DHCPNACK message).

Windows cannot obtain the domain controller name for your computer network. (The specified domain either does not exist or could not be contacted. ). Group Policy processing aborted.

Windows cannot query for the list of Group Policy objects. A message that describes the reason for this was previously logged by the policy engine.

Automatic certificate enrollment for local system failed to contact the active directory (0x8007054b). The specified domain either does not exist or could not be contacted.
Enrollment will not be performed.

Automatic certificate enrollment for local system failed to contact the active directory (0x8007054b). The specified domain either does not exist or could not be contacted.
Enrollment will not be performed

Server:
The DNS server was unable to complete directory service enumeration of zone sasinc.org. 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 extended error debug information (which may be empty) is "". The event data contains the error.

The DNS server was unable to complete directory service enumeration of zone 2.168.192.in-addr.arpa. 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 extended error debug information (which may be empty) is "". The event data contains the error.

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 extended error debug information (which may be empty) is "". The event data contains the error.

The DNS server has encountered a critical error from the Active Directory. Check that the Active Directory is functioning properly. The extended error debug information (which may be empty) is "". The event data contains the error.

What I am looking for is somewhere to start. If anyone has any ideas please let me know!