you need a internet connection only when you run on local port such as 192.168.1.1 : 50000 but even then it should not need internet but a firewall port opened for C2 to run, instead of localhost:50000 if u use the 192. port just change the settings for preview on localhost as it is default C2.
another thing that can cause that, is having java script outdated or flash player plugin the bugged one in beta that is now latest(pepper flash) its kinda creating problems i had same issues a few times with my previews its possible also to be the browser not being updated . hope it helps. but as Ashley said it shouldn't need any internet connection since its running on local host, that is your default network connection.