|Sorry, I missed that.|
Test it maybe with two computers. The adapters come with software that helps setup and show quality of service and speeds better. It may offer a clue.
See also taskman for link speeds. It may be that other data is taking up too much headroom.
Also the packet size might be too small. Might have to view with wireshark or get detailed link information another way.
As above, the speeds tend to be slower than rated. Some are as low as 6M but up to 200M or so.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10