I tried video processing on the receiver at 720p, 1080i and 1080p, but i have not tried changing the cable box.

With the box connecting properly, it works 0% of the time. power cycling manually and unplugging- replugging HDMI cable doesnt help.

I'm not going back until I have a few options to fix it. I'll bring a splitter with me next time and try that.

I did notice that when i was holding the HDMI end coming from the box and touching the Monitor out HDMI end with my other hand, there was noticeable voltage. I did it by accident and it was shocking.
Maybe there's a voltage issue with the new Xfinity boxes?