Has been happening with me as well and have read other users with the same issue. It doesn't happen a whole lot, but when it happens it's a little scary.
I'm willing to wait for a fix but I'm a bit worried as to if this can damage my TV in any way. When it's severe the tv even displays information again (for example, HDMI 1, 1080i) is the info mine displays and that usually only comes up when I swap inputs or switch channels.
I'm paranoid to get another RNG 200N because the last 3 I got had some pretty big issues and this one so far is working out fine, apart from the 'blinking'.
Sorry about the issues you are experiencing with your cable box. If you have checked all your cable connections from your TV, our cable box and including any splitters you may have and they are tight and secure. I feel as though it's your box and we should look into getting you a new one especially if this doesn't happen when you watch DVD movies. There are three options for swapping.
1. Take to a comcast service center.
2. We can mail out a replacement box to you.
3. We can schedule a technician to come out to your house and replace your cable box.
Please let me know which method you would prefer.
As Rog suggest you may get some relief by using the component outputs from the box but I think you'll still get a white line flash across the screen happening at the same interval your black screens are occurring which is still annoying.
I assume you're using a Cisco Model RNG 200N which universally has this problem. A software fix has been promised in November....maybe.
I doubt your Sony TV is the problem, I fought the same problem till I figured the problem was the RNG 200N. After going through 2 RNG 200N's I went to the local Comcast office and asked to swap it with a Scientific Atlanta (Cisco) 8300HDC which doesn't suffer any of the 200N's problems and works flawlessly. It has a bit less storage time but it won't drive you crazy with the blinks and flashes.
I have the Cisco HD DVR, i just started with comcast and every time im watching a program the tv cuts on and off "blinks" in every channel HD or regular, i know is not the tv because when watching DVDs or connected to my laptop it does not happen.