The ISP insists it's not a router/bandwidth issue. I remain unconvinced. It took hours to backup a 22-gig database file from SI's new db server to our new webserver. Both are public machines going through the same router.
On the other hand, we've had IIS occasionally just shut down with the only option being a machine reboot. While we were having that symptom, we also couldn't download ZIP files from the webserver at any time. The latter symptom is gone, and IIS never did shut down today. We'll see how it goes next week.