VPN Outlook Clients Disconnected from Exchange Server

We have several sites that all connect to the main site through an IPSec VPN tunnel over 1.5 mbps DSL. The latest site we set up is having problems not being able to login and stay logged in to the Windows Server 2003 Active Directory Domain in the main office. They're on similar hardware as the other offices, and have had both the T1 provider and ISP verify they're not seeing any errors on the lines.

The clients are all on Windows XP running Office 2003. The mapped network drives will drop for a second or two and come right back now and then through the day, but Outlook will give a "Disconnected" status and not ever reconnect. Users experience the issue at different times during the day, sometimes it works fine, sometimes it doesn't work at all. The issue occurs even when only 1 or two users are connected, so I don't think at this point it's any sort of traffic congestion.

When Outlook disconnects it gives the following errors:

Task "Microsoft Exchange Server" reported error (0x80040115): "The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action."

Task 'Microsoft Exchange Server' reported error (0x8004011d): 'The server is not available. Contact your administrator if this condition persists.'

When the user first logs on, these events are logged in the Event Viewer:

Application
Source: EvtEng EventID: 0 Type: Information Description: The description for Event ID ( 0 ) in Source ( EvtEng ) cannot be found....

Source: Userenv EventID: 1054 Type: Error Description: Windows cannot obtain the domain controller name for your ocmputer network. (The specified domain either does not exist or could not be contacted ). Group policy processing aborted.

System:
Source: Netlogon EventID: 5719 Type: Error Description: No Domain Controller is available for domain {DOMAIN_NAME} due to the following: There are currently no logon servers available to service the logon request. Make sure the computer is connected to the network and try again.

Despite the error messages that the Group Policy was aborted, a gpresult shows that the policy has been applied. Some users are wireless and others wired at the remote site. The setup is as follows:

Remote Site Users --> Wireless router or Wired Switch --> Netscreen 5GT Firewall w/IPSEC VPN Tunnel Established --> 2Wire DSL Modem in Transparent Bridging mode --> Internet --> Main office Netscreen 25 --> Windows Server 2003 Active Directory Domain Controller

All the other sites have identical hardware and configuration, no one else is complaining of this problem. The firewall logs show the tunnel is not being dropped.

In troubleshooting so far, I've:

1.) Configured a lower MTU (1500) for the DSL facing interfaces of the workstations
2.) Configured Kerberos to use TCP instead of UDP
3.) Switched out the Wireless Router and the Wired Switch
4.) Have had ISP and T1 provider both verify they see no errors.

Am stumped here, ready to give Microsoft a call, thought I'd put it out here to the community first.