Outlook get disconnected

[selina]

I'm having 1 or 2 (depending on if they are related or not) problems on Exchange 2003 and/or Outlook 2003.

[Problem 1]
Only in Branch Offices
Outook loses connection to Exchange server and stays disconnected for about 2 hours at a time.
The time uses' connection to Excange server is random.

Situtation:
- Ping to exchange server by name returns normally
- Able to create a new outlook profile
- Cannot open Outlook using the newly created outlook profile (Error: "Default mail folder could not open. Outlook must be online or connected to Exchange Server....")
- Able to open Outlook using existing outlook profile but cannot complete Send/Receive (Error: Task 'Microsoft Exchange Server' reported error (0x800401105): 'The connection to the Microsoft Exchange Server is Unavailable. Outlook must be online or connected to complete this action')

- Changed the size of packets goes through Firewall to improve the flow of traffic > no vail
- Copied PST file to the local computer from the server > no vail
- Disabled Windows Firewall in Exchange server. no vail
- Changed GPO link order from AD > no vail

[Problem 2]
Only in HQ (where exchange server resides)
Outlook become slow every now and then and freezes when...
- Switch between folders within Personal Folders
- Click on Send/Receive
- Attaching files to a message

Situation:
- Access to Public Folders is normal
- Access to Mailbox is normal
- If defaul mail delivery location is NOT PST file, the Outlook performance is normal
- If default mail delivery location is PST file which is located in the local HD, the Outlook performance is normal
- Most of users who are having this problem are using PST file which is larger than 1GB.