I've had the same problem in the past and I tend to blame it (maybe incorrectly, I dont know much about this stuff) on the cable modem. When I first got the service they gave me a cheap generic modem. My connection would drop frequently but I just had to unplug and replug the modem to get it working again.
Eventually the cheap modem got fried by a lightning strike at which point they replaced it with a Surfboard 5100. At the same time though, they replaced all the connections on my cabling, with the cable guy claiming these connections had insufficient sheilding or something and they were required to replace them by the FCC. In any case, my service very rarely drops now. It only seems to drop if I leave a bunch of bittorrent streams going overnight. It can still be fixed by unplugging/replugging the modem.
It seems you already have a decent modem though, so I don't know what to tell you.