This sounds very much like a problem I had a few years ago.
This has been TRUE for me too for the past 1 1/2 yrs - but as of 9/1 I have started getting DD, loss of synch issues, on ABC, CBS, NBC, and FOX. I use the 24/500 DVR Receiver and it is and has been set on DD all this time. Using HDMI Cables I/O.......tried also installing an Optical/Cigital Cable yesterday to see if that would change things - BUT it has not. DTV tells me it is on my end..........but "common sense" tells me different......because I ONLY get these irritating drop outs of audio [split second to 5-15 seconds] while watching those major networks. IF it was on my end I would think that ALL their channels would be acting this way.
HATE to go back to Comcast...........but in a few more months now I have to consider it...........YUCK!!!!!!!!!!!!
I'd suggest emailing firstname.lastname@example.org
Explain the problem and how it's only with your local HD channels.
One of Ellen's group should contact you.