Ubuntu 12 - Making My Gigabit Lan stay Gigabit

Today, I discovered that my file transfers were getting capped at 100mbit instead of 1gigabit. I discovered that if I plugged in a different cat6 cable, it goes back up to 1000mbit (gigabit). Initially I thought that I had been duped on my cat6 cable. After further testing, I discovered that restarting my computer would result in a gigabit connection for a period of time before dropping down to 100mbit again. The speed can be found by running this command:

sudo ethtool eth0

(ubuntu doesn't come with it by default so you may need to do a sudo apt-get install ethtool). Below is a screenshot example of the output with the speed highlighted.

For now, I have 'solved' the problem by manually setting the speed with ethtool:

sudo ethtool -s eth0 autoneg off
sudo ethtool -s eth0 speed 1000 duplex full

I can only put this down to an error in the 'auto-negotiation'. I read that this will reset each time I reboot, but editing the /etc/network/interfaces config file as suggested by a site only resulted in my network no longer working, so I will just run those commands in a script every time I reboot.