Boot machine, login in to OL2003 and check email with no problem.

After surfing the web for a while, go back to OL2003 and try checking mail and get the error number in the title and a message similar to
Outlook is unable to connect to your incoming (POP3) e-mail server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP)
.If I reboot the problem goes away for a while.

I've tried timing it to see if if it's a time thing, tried after cetain sites, using certain programs, updating Ad-Aware, Spybot, A-Squared, NAV etc.

I can't seem to pin point when it happens, but as I said a reboot seems to correct it unti lthe next time.

What to do ?