Just for ha-ha's, here's a few things to try:

- The computer in question... Try dragging it over to the destination hub/switch/whatever and try plugging it directly, using the same cable. Does it work?

- If not, try using a crossover cable. Does that work?

- Check the pins on the jack itself. Sometimes people plug telephone cords into network jacks. This bends the outer two pins badly and makes the connection work intermittently.

- In-wall and cable length? Not supposed to exceed 326 feet.

- Negotiation speed of NIC and Hub? Is it a 10/100 incompatibility? Even if both or either is supposed to be autosensing, I've had that kind of failure before and I was SURE it was a cabling problem until I throttled the NIC back to 10mbps and suddenly everything was fine.