It sounds like you may want to check your upload and download speed. You can do this by going to speedtest.net. You want to be atleast at 1.5 mbps.
Here is an article that provides all the system and network requirements. Slingbox PRO-HD system requirements for HD and SD streaming. If it still gives you the same issue, you may want to run the setup again by resetting your Slingbox and going to setup.slingbox.com.
Sling Media Forum Moderation Team