I'm seeing the same behavior - it only occurs when I am downloading data at a rate of around 2 MBps or faster. First the rate decreases, then I eventually lose connection to Cox's DNS servers until I reset my modem. This started about three months ago. It doesn't matter where I'm downloading from, be it Steam or iTunes or wherever.
I can otherwise use my Internet connection with no problems at all, ever. But once I begin getting good speeds my connection will be throttled and eventually severed. It's really disappointing. I'm moving at the end of the year and if the problem still persists I'll just be going with Verizon. It will be a little more expensive for a lesser connection, but at least they won't throttle my connection.