If you do, check out his internet sessions with the router in place. Maybe you have a virus or something on your network that is creating too many sim connections, that when in conjection with your PC doing a big download, it crashes the modem.
I would also check the logs of the modem. Its possible that its happening when the router is there or not, but when the router isn't there it's not actually dropping connection so your not thinking to look at the modem.
Those are my two guesses. Furry, let me know how it goes. Could also be a firmware bug in the modem.