I have a SB4100 and have the same problem - I can force a modem reboot if I visit the messageboard on
www.b3ta.com.
The thing is, this page contains a lot of external links to images on other sites - my suspicion was that it was either an overload of DNS request or perhaps just an overload of UDP packets going via the modem. It's not a general problem with large numbers of connections as P2P type stuff (e.g. BitTorrent) works fine.
This has only started in the past few weeks. I'm guessing that it's not the local modem but more likely the head-end equipment that has been 'upgraded' at some point.