I would definitely try Paul's suggestion above; however, if this does not work, I wonder if you are running into Selectable Output Control (SOC) issues. SOC is a software within your cable box that block video streams when there is an HDMI cable running. Even if you just have an HDMI cable running from your set top box to the TV, it will still detect the Slingbox on another input (for example, component) and, since this is a media device, it will assume you are trying to record and distribute the stream. The only way around this would be to remove the HDMI source, run component only (which gives you the same video quality as HDMI), and then power cycle the cable box.
I hope this helps!