I have to assume you have a HDTV and are connected with a HDMI cable. Your error indicates a intermittent disconnection between the TV and the cable box.
If you haven't power cycled the cable box, do that first. Unplug the AC power for two minutes, then power it up again. Also confirm your HDMI cable is firmly plugged on on both ends. Power up the box, and give it a test.
If it still does it, then next step would be to take the cable box and remote to the local Comcast office for a swap. You can also contact Comcast and insist on a tech visit if you want to wait for one to change out the cable box.
Some people with this issue sometimes gets it resolved by switching from a HDMI cable to component cables, Red/Grn/Blu for video and Red/White for audio.
I've found that over time these Comcast cable boxes can slowly degrade. One day you might experience an occasional flicker in the picture or sound, but days, weeks, or months later you may find the picture or sound failing repeatedly.
You might notice too that the image brightness and contrast diminishes over time.
In my case, I believe that my Comcast box's poor operation was aggravated by its operating heat, so whenever I unplugged the box for a few hours and then plugged it back in, it would work again for 24 hours. ...Then it would begin to fail all over.
I finally took the failing box to the Comcast Service Center and replaced it.
Comcast cable boxes sometimes come to an end, not with a bang, but with a whimper.