:(

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?

Yes It does work. I will try a crossover tomorrow with the wall box.

- 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.

Jacks are new, have only been in my possession and I have checked them and they are fine.

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

About 30 feet total.

- 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.

10 Mb card going to a 3Com superStack II Autosensing Switch. Works fine when the Wall box isn't in the equation.

Did you wire these cables yourself?When you do that, you have to use a certain pattern of wire pairs, and there is a certain pair that need to be intertwined to prevent crosstalk.Sometimes a short cable run will tolerate incorrect wire-pairing, but it will begin to fail on long cable runs.

Yup, I used the 598A (iirc) standard and I also used whiteblue/blue whiteorange/orange whitegreen/green white brown/brown and I also used ....

This bites, I will try a crossover cable tomorrow and failing that I will break out the meter and trace the circuit boards to see what pin is going to which one in the connector. If it looks totally screwed up (as it did today with trying to follow the traces manually without a tester) I might just wire the wall box myself with a few wires and a bit of solder.