Comcast made themselves look worse with the "Xfinity" branding. Every week I have some kind of issues with their crappy DTAs that still don't get all the channels I once had with analog (i.e. Weatherscan, truTV). They really should ditch the DTAs and go back to analog. Sure it uses more bandwidth, but who cares, it's a lot more simple and convenient. At least analog cable doesn't frequently pixelate or have its weekly "service unavailable" crap. And please, ADD MORE CHANNELS TO QAM! 
The people I talk to over the phone seem to have no idea what I'm saying to them either when I ask them about QAM or their crappy DTAs.
I hate to break it to you, unfortunately, but it may have to do with the strength of the signal coming from your cable line... If you have excessive splits going through the line leading to your DTA, your signal's not going to be strong enough to grab some of your channels. The DTA
only displays the "service unavailable" screen only if it cannot lock on to a signal for the specified channel. I would have Comcast out to look at your line or remove the excessive splits. Your choice.
