I've noticed that when I first log in to my SB account to watch my Slingbox, it "initializes" and looks for the box. Perhaps this involves going out to the slingbox.com website (on the WAN) to acquire the box ID and IP addresses? But, once it is running, the speed is much faster when I connected with a computer on the same LAN than from a computer outside the LAN. So maybe the WAN connection is no longer necessary after the streamingstarts.
You might test whether you need the WAN to connect to the SB on your LAN by temporarily disconnecting the WAN (for example, by unplugging the power on your modem) and see if you can still launch your Slingplayer software and connect and stream.
Or, unplug the cable modem AFTER you have already connected and see if the streamingcontinues normally after the WAN has been disconnected.
I have a similar problem and wonder if you can help me. When i go to the website slingbox.com to stream video, it initializes and then starts to stream fine. Then in a couple of minutes the picture freezes and goes black and I only have sound. This happens on 4 computers and in safari AND firefox AND chrome. However 2 of those computers have the onld icon for slingbox -- the one you click on and it goes immediately to streaming. The icon works perfectly!!! But the www.slingbox.com doesn't. Has anyone had this problem. Tech support has been very UNhelpful!!!!