Its likely powerboost doing that. Try downloading »ftp://ftp1.optonline.net/test512 for about a minute or two and see what your download speed is at.
When I download Linux distros, I run at 25, then powerboost gives up and I have a solid 19 down. Upload runs up to 4.5 for before settling at constant 2.5.
Here's a screencap for the test file you linked.