Technomate 5403 - sound problems

I recently aquired a Technomate 5402 (replaced a GI Mini for intelsat 907). It really is streets ahead but I do seem to have a small problem. Occasionally I have sound problems on some HD channels (both HDMI and optical output). It really mainfests itself on film soundtracks, dialogue is ok. It sounds like the music score in some films is underwater.
Anybody any idea what I could try?

Thanks