Hmmm...
This is starting to sound like a hardware issue. Make sure you have the latest drivers installed for all your network hardware.
That is to say, (assuming you're running windows), Start->Run, type in "cmd" (no quotes), press enter.
In the Command Prompt, type in "ipconfig /all" (no quotes) and press enter.
Google all the names that pop up (adding the words "Driver", "Download", and whatever version of windows you have).
Me:
2.99Mbps Down
0.96Mbps Up