Welcome to the world of analog on a digital set.
Use Component from the STB (Set top box) to the TV. Component is Red Green Blue for video and Red and White for audio. The Xbox is using Composite (Red, Yellow, White; Yellow for video Red and White for audio).
Guess I need to read the post fully.
Sounds like ghosting... need a picture to verify. And what type of connection is going between the STB and the TV? Does it happen ONLY on the STB or the Xbox as well?
Not likely to be any help because of where you live.
However one thing I noticed is that SCART is analog not sure how it is in comparison to coax though...
If you have the option of using Component (RGB+RW) then use that instead.
Guess not...