If your Hi-Def Tv is running through a reciever, have you experienced a few second picture (but NOT sound) blackout when it goes from Hi-Def to like a non hi-def commercial? Or switching between a hi-def channel and one of the lower regular channels. During this there will be a box in the upper left hand corner that says No Signal or invalid format, this all happens really quickly. Had an earlier issue that was resolved with a new cable box (I'll include the thread link, it lists my componants).
http://www.texasguntalk.com/forums/electronics/21942-sound-cut-out.html
http://www.texasguntalk.com/forums/electronics/21942-sound-cut-out.html