1Gbps network interface, 1Gbps switch, all of a sudden the card refuses to play at anything more than 10mbps. The fix, turn the power off for 15 minutes. The explanation which I gleaned from an obscure thread on an even more obscure message board, the
PHY chips in your network card can go a bit nuts in response to power fluctuations and will retain their settings thanks to WOL keeping them powered. Power off for long enough, all is well again.