Picture breaks up when music is played on m Humax 9300T?

I have a Humax 9300T set- top box but when I play programmes that I have recorded the picture breaks up i.e when music is played on the X Factor or music at the beginning or end of programmes or at intervals in programmes. It works fine otherwise, it is only on recorded programmes when music is played. Why is this and how do I fix it?
Thanks.