SKY+ optical output problem

I have the version 2 Sky+ box and am using the optical output from this via a high quality IXOS lead to my amp/dolby digital decoder. unfortunately even though optical output is turned on from the box expecially on movies 1,2, and 3 when the channel is selected, there is an approx 2 second delay before the sound is audible. Furthermore i can hear an intermittent low freq pulsating thud noise gently in the background. Occasionally no sound is audible when the channel is changed. The problem usually resolves by resetting the sky box. Any bright sparks out there with ideas? Many thanks