Sound problem

I am running an explorer 3250HD cable box and I am using an optical audio cable from the cable box to my Aiwa receiver. The problem I am experiencing is that the receiver will momentarily switch from the digital signal to analog. This will cause no sound for just a second, but it is kind of annoying. The light on the display will switch over to analog and then right back to digital. all my connections are tight. Could this be a defective optical cord? or might there be another issue here.