The Intel 82579LM in my Dell PC claims to support gigabit speeds, but it maxes out at around 250 Mbps. Google turns up lots of other people with similar problems. I ordered an $11 gigabit PCI card from Amazon.
https://www.google.com/search?q=Intel+82579LM+slow
https://www.google.com/search?q=Intel+82579LM+slow