Why do my Panasonic surround sound front speakers keep cutting out?

I have a brand new out of the box Panasonic SC-BTT490 3D Blu-Ray Disc 5.1 Surround Sound Home Theater System connected to a 50" Panasonic Vierra via an optical cable. The front speakers (left, center, right) keep cutting out on me while the sound in the 2 rear speakers are not affected. I notice it only does it every now and again on certain cable channels. Is there a setting that I may have missed on my TV and/or surround sound? Please help, it is really aggrovating !!!
@Kevin -
Now that I think about it, they don't cut out when the Xbox, dvd, or blue ray is playing. The optical cable seems fine. As I am writing this I haven't heard it cut out one time since having my cable and surround sound on a different channel than before. Im going to agree with your diagnois on this and narrow down the culprit to certain channels on the cable box. I will be contacting my cable subscriber about this issue. Thanks for your explanation, I appreciate it.