Quite apart from the fact that my own gigabit got sick for a while, I have found some inconsistencies in gigabit connectivity.
Specifically, the Mac Mini will negotiate Gigabit with some of the ports on my network switch, dropping back to 100mbps on others. The windows PC, as well as other half's work lappie, are happy with gigabit on all ports, and will achieve transfer speeds to prove it, well over 300mbps sustained for disk to disk transfer.
My XP box has a clever trick, if you can find it in device manager, and that is the 'cable test' function. It worked well, and not only told me I'd messed up but accurately estimated the distance to a cable fault I'd created when wiring the house a few years ago. Can't remember what the chip in that box is, but it was a neat trick.
