I have had the same problem since I've been using my OTA antenna for local HD broadcasts and I am getting a great signal.. Approximately 95 percent give or take. Most of the time its just an audio drop but sometimes picture also.
It is my understanding that CBS here in Chicago, uses a lower VHF channel, (3) to be exact for their Digital HD broadcast... I have been told that the lower VHF channels are more susceptible to interference and they are harder to push. For example, if I run my microwave oven or simply flip a light switch on or off I lose audio for a second or two and sometimes even get pixelization. With the microwave running, I cant even watch the channel.. I currently live in a townhouse so I am assuming that whenever anyone else in the building uses anything electronic I get the same affect.
I notice the audio drop outs way more often while using the Digital 5.1 out to my stereo receiver. I am using a Hughes HTL-HD DirecTV receiver.
Hope this helps...
Luckily I also receive the New York CBS HD feed through DirecTV so I switch over to that when it gets bad...