Thank you for utilizing our Slingbox answer forums. We are sorry for the problem you are having with the Slingbox PRO-HD and not being able to complete the Internet Viewing test. Based off the information you provided, at 80% the Slingbox is restarting and if you are getting the message that the Slingbox was disconnected do to a poor Internet connection that means the unit was not able to come with a network connection fast enough. When this issue is occurring, if your Slingbox is not hardwired directly to the router I would try hardwiring the Slingbox to router. If you are hardwired to the router with your Slingbox, and your computer is wireless to the router, try hardwiring the computer to the router as well.
I would also like to provide you with the article below. This article will help you with finding out what is causing the issue:
Sling Media Moderation Team