Try waiting for two minutes or more before restarting--service should resume. A couple of other ways to help ease the transition--use file->disconnect a few minutes before quitting (uploads and downloads will still continue), and you can also quit all Internet apps to help.
Especially if you were running as an ultapeer, some cablemodems and the OS can take a while to terminate all the interrupted threads--which seems like a freeze. (my ISP eventually provided a DOCSIS cablemodem that doesn't "freeze" on me anymore).