I think this problem is simplier than you think. Most gigabit adaptors, especially cheaper ones, do not give you the option to force a gigabit connection. You can either select 10, 100 or auto. It has to use auto for gigabit. Do you have any indication that it is not connecting at gigabit speed? Try using some network bandwidth testing software. Or, a simplier test is to force it at 100/full, copy a large file and time it. Then set it to auto and do the same. See if the file copies faster.