Outlook is disconnected

I have a number of users who are having problems connecting to our Exchange
Server when they open Outlook 2003. This happens frequently, but not everyday
and not always to the same users. They never get any kind of error message,
just the 'disconnected' notice down in the bottom right corner of Outlook.

We've tried reinstalling Outlook, re-creating their mail profiles, restoring
the connection through "Connection Status" and the problem keeps coming back.
The only thing that seems to work, at least for the short term, is to search
for and delete all the .tmp files on the local pc's and reboot them. The
problem may or may not come back the next day, but will eventually return.

We're running Exchange 2003 on our mail server and fully patched XP sp2 with
Office/Outlook 2003 sp1 in a small network of 90-100 clients.

Does anybody have any ideas what is causing Outlook/Exchange to behave like
this? My users are getting tired of me telling them to "shut-up and reboot"
all the time. There has to be something I'm missing, I just don't know what!

Thanks in advance for any help you can give me=0)