TV Blacks out; Mode not supported message on TV screen.
One TV was intermittently blacking out. Sometimes I would get a screen message saying "Mode not supported". It was driving us nuts. I replaced the cable box 4 times, swapping tvs with another room, talking with technical support and having 1 service call confirming my signal to the house was very good without a resolution. Then it finally hit me. What haven't I switched out? The HDMI cable that was supplied by Xfinity. I went to the orange box store and bought a "deluxe" cable; not the basic one. EUREKA. Problem solved. After 4 days, no blackouts or messages.
If you are having similar issues, try my solution. You can always return the new cable.