Thanks for posting to our Forums! I'm sorry to hear that you are having issues with losing you video on you Slingbox, when you plug in the HDMI cable into your cable box. The reason you are losing video is you are running into what's known as SOC better known as Selectable Output Control. To get around this you would simply need to disconnect the HDMI cable from the cable box and run Component cables out of the Slingbox into the TV. You will not lose any video quality running component cables and you will never have to worry about loosing your video again
Below is an article that will assist:
Hope this helps!
The Sling Media Moderation Team