OK then, here's the problem explained:

* Ethernet works fine on a fresh reboot after plugging power in, and continues to do so.
* Reboot into Windows 8. Ethernet works fine with no fiddling.
* Reboot into Linux. Ethernet needs me to poke MTU down to 1496 instead of default 1500.
* Unplug computer, and reboot. See 1.

lolwut? I have a SiS191 ethernet chipset inside of this.