I have had similar issues with my own Xfinity DVR and client boxes running through Yamaha AVRs.

I get a blue screen on the TV about 25% of the time I power up. I can generally fix this by manually turning the DVR on and off (manually since there is no power off on the Xfinity remote). If this doesn't work, I have to remove the HDMI cable from the DVR and replace it.

I believe that this is an HDMI handshake issue, but cannot prove it. The strangest part of this is that all of my Xfinity boxes worked fine on their respective Yamaha AVRs until about 6 months ago. I suspect some kind of Comcast firmware update, but again, I cannot prove it.

One of my client boxes failing to connect about 99% of the time, so I added a cheap HMDI 1x2 splitter between the client box and the AVR. Per several sources, these splitters will often strip out the handshake. This fixed the problem immediately.

I have not tried this on my main DVR box because it works about 75% of the time and when it doesn't work, the fix is easy (and I am cheap).

Not sure if this is the same issue causing your problems, but I would try adding a splitter. Here is what I used:

[Link: amazon.com]

Good luck!