I get picture breakup and audio dropout with Discovery HD, INHD, and TNTHD. Discovery has been the worst of late. Which is odd because when I first got it in the beginning of June it was the least likely to have problems. ESPN2 HD had just come on and was terrible, but is rock solid now.
Of course, for me it's primarily due to signal strength. My signals waffle between 28-35 SNR. Closer to 35 is great. Around 30 and below, I have breakups every minute. Discovery is usually around 30, unfortunately.
I've had a truck come out. They replaced the splitter right outside my home, and said they need to do something with the line/connection at the street but haven't done anything since. It's been weeks; I gotta call them again.
Edit: I have Comcast in Maryland (Anne Arundel Co.). Verizon just laid down fiber in my neighborhood. I'm waiting patiently for FIOS TV to come up so I can take a gander.