It is happening on other channels - you just aren't watching those (could be sports or shopping ones etc).
this is a classic sign that your box is on its way out - I'm on my third one in ten years, though others never seem to have a problem!
That interference will spread over time to more and more channels as the box dies.At some stage you'll need to change it, and that may cost a call-out fee.
Your dish is possibly experiencing some interference too, check your signal strength and quality - got any trees nearby?
When it all becomes too annoying to ignore, and the sky man comes, get him to change your box and check your dish too - all included in the cost.