Realtek onboard LAN doesn't work above 10 Mbps

Issue affects Windows 7, Windows XP, and Ubuntu Linux.

As of yesterday, my onboard Realtek Gigabit Ethernet NIC stopped working
fully. It's a "Realtek PCIe GBE Family Controller", model RTL8167. It
was the NIC built into the motherboard of my PC, which is an ASUS
M4A785-M. Initially, it wasn't working at all when plugged into my
router, so I plugged it into a separate hub, and it somehow got working
at the lowest possible speed of 10 Mbps. This NIC has previously worked
both at 100 Mbps, and 1 Gbps!

I also tried this under Ubuntu Linux, thinking maybe it's a Windows
driver problem, but the same problem occurs under Linux, so it's looking
like a real hardware problem. Perhaps it's an autodetect issue, is there
some way to hard-code it to run at 100 Mbps or 1 Gbps, without going
through autodetection?

Yousuf Khan