Strange Network Card Issue (not showing in device manager)

Question

First of all, apologies, I posted this on ServerFault earlier but realised it'd probably be better here.

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A friend asked if I could have a look at one of their computers because they kept losing all network traffic (local and internet).

Unfortunately I forgot to write the motherboard name, but it was an onboard NIC (wired ethernet). Anyway, it wasn't showing at all, nothing in network connections, nothing in device manager etc... so I thought maybe it'd been disabled in the BIOS somehow. Checked but it was enabled in BIOS.

Tried reinstalling the drivers and did a few restarts and it started working again, but a week later the same thing happened.

I thought it must have been faulty so I bought a PCI NIC and set that up. All working fine for a couple of weeks but now the same thing has happened again.

I've never dealt with anything like this before, just wondered if anyone's got any suggestions?

The PC is running XP, however, it's connecting to the internet through another PC using ICS. But I assumed if it was a problem with that, I'd at least get a "Network Cable is Unplugged" message... and it'd still show in Device Manager.

Thanks

Update - 2010-09-14:

A bit more information on this, strangely, no matter how many times I restart it doesn't fix the problem... but as soon as I actually turn off the power at the plug and leave it for 30 seconds, the network card is recognised again and works fine the next time I start it up.

So as mentioned, it's surely got to be a hardware issue when removing power from the system is changing the outcome... it's just how I'm going to find out which part is faulty now without randomly changing bits :\

Answer 1

I would guess, based on what you're saying, that it is most likely a mainboard issue. There is nothing else on the system that would cause a failure for both an on-board component and a PCI component. The only other possibility I can think of is the HDD, if sectors affecting portions of the network drivers or protocols is damaged, but I would think you'd see some crashing rather than network failure if this was the case.