This is very normal. Nothing is wrong with your cablemodem.
When downloading, you are still uploading acknowlegement packets to the sender. There is no escaping this with TCP connections... it is part of the design to make sure things are transferred in the proper order, etc. To make things worse, DCC also sends an ACK back to the sender, so that uses more of your upstream (this is the main point about why DCC is a poor protocol for file transfers).
If you are using all of your upstream uploading a file to somebody, then there is none left to send ACK packets back to the sender, so eventually you start receiving the files slower. This is horribly noticable on cablemodems, since the modems have their own queue but it is large and very inefficient.
If you'd like to learn more about this, google for "Quality of Service" or "bandwidth throttling".