Then in all likelyhood this is a Comcast issue. My Comcast connection varies it's upstream speed anywhere from 25 to 8-9Mb on different days - they put much less emphasis on that part of the system since first, it's more difficult to get high upload speeds with a shared-media DOCSIS system, and second, they don't want people doing lots of uploading anyway, to discourage things like web sites and PtP sharing. HTTP requests don't take much space...
Jim