Your problem here seems very similar to the "Comcast Denver ...." post from sbwinter2.
I think it may have something to do with Comcast moving along with phase 2 in MPEG4 project. You might read their post found farther down on page 1 of this forum.
This is just my opinion. I have not heard anything on this subject.
This has been an on and off issue for nearly a year now. It is anamorphic (stretched) for a few days, then goes back to letterbox, then back to anamorphic, etc... It's not just History and A&E, but also several other networks.
It's not a DTA issue, but an SD channel issue. You can see it even with a normal cable box.
I think the SD copies are just Comcast downconverting the HD feed at their master site where they pack everything together for national distribution. For some reason, whatever box is doing the conversion for those channels switches itself to anamorphic instead of letterbox.
Here is a post I made back in June:
Thanks for posting here about your reception issues on your DTA box. I'd be happy to try and send a signal to your box in an attempt to resolve this.
As far as MPEG4 is concerned, we've been rolling it out all year and are looking at covering a few remaining areas by the year's end.