Thanks for posting in the forums! I'm sorry to hear you are not able to get a stable video signal when connecting to your Slingbox PRO-HD.
It sounds like this could be Selectable Output Control (SOC). If you are using Component cables, when connecting an HDMI cable to your set-top box, it could cause this issue, or even cause you to not get a video signal at all. A way to test this is to disconnect any HDMI cables that are connected to your set-top box. After this is done, you should get a clear picture.
Below is a link to an article that explains SOC in better detail:
Feel free to reply!
The Sling Moderation Team