Is your set-top box also connected via HDMI? That could be just how your set-top box employs digital and audio outputs simultaneously. Unfortunately, different set-top boxes will show different symptoms when HDMI is used.
Otherwise, I would recommend trying to setup another input like S-Video or Composite. You might be able switch inputs to have it reinitalize the component video source. It could be an issue with how the set-top reintializes its component outputs when it comes out of stand-by.
Thanks for your reply, eferz.
The only connection going from the set-top box is component + red/white for audio to the slingbox and from the slingbox using the same connections to the TV. The HDMI on the set-top box is not connected to anything.
The only video outputs available on the set-top box are HDMI, component and scart.
If I understand your suggestion you want me to try connecting the set-top box with the slingbox with the scart output at the same time, is that correct?