I had a similar issue with wifi with my MBA, only getting 1-2 Mbps down and 700kbps up with Comcast. I went into Preferences, deleted my wifi connection, rebooted and reestablished my home wifi network. Now it pops around 25 Mbps down and 4 Mbps up. Maybe you should try the same thing with your ethernet connection.
What are you getting via wireless on that same connection? If wireless is just as slow then it's not an adapter problem. If wireless is significant faster that would definitely indicate a potential adapter issue but you need to establish a baseline. Preferrably not a speed test through a website, either. An actual file to serve as your "test subject."