In reply to:
You might try configuring a static IP address on the PC to eliminate DHCP from the equation. You need an address in the router's subnet, and you need the default gateway to be the IP address of the router. I think the DG834 router also acts as DNS as well, but if you try connecting to the config web page then that doesn't matter for now.
Forget about gateway at this stage. I concur, otherwise, the config page is enough.
The loss of MAC address on the mainboard could be the problem. Might be worth getting hold of a network card and trying that in. Might also be worth a check in the bios to see that on board ethernet is at least enabled. [Even if it is on, I might turn it off, reboot and turn it back on, just for the hell of it]
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