No real answer(s) here..?
A friend picked up an S3 this week and I went over to help him set it up and he's having similar issues.
All SD channels have fine picture and audio.
50% of his HD channels have fine picture and audio.
50% of his HD channels simply have a blank screen which, after over an hour of tweaking, we couldn't get to display a picture (trying working through all the various output settings, fixed, hybrid, native, etc).
Ultimately, he gave up and plugged his 6412 (Comcast customer) back in and enjoyed all his channels.
We'll call Tivo/Comcast today, but it seems other folks have had this issue and there's no fix..?