Exchange Server, Outlook, VPN, failing....

I have a problem that I am having a HELL of a time trying to solve.

We have an dedicated exchange server hosted offsite, using a site-to-site
VPN to connect to the office. All computers in the office are configured
similarly. Exchange 2000 connected to Outlook2003.

One of the laptops is coming in and out of connectivity. I can see her
coming in and out, on the laptop it is showing "connection to exchange server
restored', but then it looses connection, showing "trying to retrieve data
from the Microsoft Exchange Server" messages. Problem is that NO DATA AT ALL
is being downloaded or uploaded, making using outlook virtually impossible.

All of the other computers in the office are configured the same way, none
of them are having this problem.

The server has been rebooted. Outlook has been 'repaired'. Outlook was
then uninstalled, reinstalled. The OPERATING SYSTEM HAS BEEN REINSTALLED.
We have no idea what to do next.

Any ideas?