Does the volume change with channel or programme changes?

It could be switching between Dolby and PCM if that's the case. Dolby has compression settings that could cause large volume changes between that and full scale PCM.

Just a thought. From your post I'm guessing it's more random, but maybe worth eliminating.

Cheers,

Mark.

861v8, DSP5200, DSP5200HC, DSP5000mk2, UHD722, MF10, Prime, Prime PSU