Nak AV-10 help

[rowe001]

Came into an AV-10 a year ago via a fortunate EBay bid of $400
One year later in a position to get surround sound set up. Bought Energy Take 5 speakers (no sub) to connect in.

Have a Song BluRay connected to the AV10 with Monster optical cable.
Have the BluRay set to optical 24 bit Digital out.

Here's the perplexing and frustrating result.

With 2 speakers, all is good. As soon As I connect in the center channel the sound stutters on/off for all 3 speakers.

The AV-10 auto-detects Dolby digital....but wht the stuttering....
Not the cable it is new....could it be the AV-10?

Thanks for all help/clues in advance

cheers all