download disconnects

This modem will surf all day long. But go to a download site to download anything and zap!!! You get kicked offline. It disconnects you. But, here’s the tricky part. You can go in and get upgrades on anything that you already have on your system. Anti-vitus updates or realplay upgrades. No prob!!! Anyone have any clues? All the internet settings seem to be the same as before this prob.