Dell :: D820 Unreliable External LAN Connection

We have a user using a Latitude D820 running Windows XP. It's a slow machine, but it's been working generally fine up until yesterday, when it started having connection issues.

When connected via an ethernet cable, the laptop can connect to everything on our internal network (fileserver, Exchange, internal websites), but anything outside (Google, other websites) is inaccessible (Internet Explorer cannot display this webpage). External sites will then be accessible after a few minutes, then connectivity will be lost in about another 5 - 15 minutes. This process repeats seemingly endlessly. We've replicated this in separate offices, so it's not the wiring. And everything works fine running off of the wireless.

Things we've tried:

Disabling the 1394 Connection
Disabling the wireless connection
Disabling all non-LAN adapters
Restarting the computer
Disabling/re-enabling the LAN connection
Updating the network card drivers
Resetting IE's settings
Cleaning out the user's temporary internet files
Logging into the machine as a different user (local administrator)

Things we know aren't causing the problem:

Firewall
Antivirus
Our network
Proxies