Here's something to consider: perhaps Comcast is not intentionally disconnecting you due to trying to use more than 130KBps upstream, but instead your network is getting timed out or something because of the amount of simultaneous connections?
When an ISP caps your speed at something like 1.5Mps, while offering you more expensive plans that allow 3.0Mps and 5.0Mps, these caps are hard limits that can't be circumvented. But dear lord if some one knows a way of circumventing these caps, I'll be your best friend LOL.
But if what you're talking about is you believe they are forcing disconnects as a type of throttling, I don't think they are actually doing that (though I could be wrong). I believe your hitting your maximum upload speed and there are too many connections trying to get through at one time, and this is causing the entire set of connections to shut down for a moment.
Something else I just thought of that could be a contributing factor is how many ports or connections simultaneously allowed by any given flavor of windows. Might want to check on that. Here is an article on it, but you can do more googling if you wish http://support.microsoft.com/kb/314882
But as another poster suggested, you may also want to contact Comcast about this issue to get conformation on whether or not they are forcing disconnects for excessive uploads, ask them if there is some way to get them to stop.
if not, the only thing I can think of is shell out more money for a faster service, ask them how much to upgrade to a faster service, or ask if there is anyway they can alter your upload/download limit by reducing x amount of download bandwidth and giving the remaining to the upload stream so that upload and download have equal maximum potential without you actually using more maximum bandwidth. I doubt they'll do this because it is easier for them technologically/economically to give customers higher download than upload. A long boring explanation of why it's harder on an ISP to give high upload than download is explained in this wiki article http://en.wikipedia.org/wiki/Asymmetric_digital_subscriber_line