Sounds like an HDMI handshake issue. Next time it happens try switching the TV to a different input and back, I'm betting that will bring back the picture.
I switched HDMI inputs and things seemed fine for about six hours. Then again, NO PICTURE after I turned the HR23 on. I also know it's not the HDMI because I sent an S-connector to another TV and the same NO PICTURE results. I took the splitter out that split the signal between the tuner and the internet. No luck there. I did an RBR and a menu reset. Still no luck. Here are some things I forgot to mention: 1. After I finally get a picture, and if I don't touch the remote for a few hours, the screen goes SCREENSAVER. 2. I recently had Weakknees change the hard drive from a 500 meg to a 1 terabyte. When I get a black screen, if I play a recorded video, the video plays fine immediately. No problem with the HR23 before the hard drive switch except occasionally for the SCREENSAVER phenomenon, but the NO PICTURE after turning on didn't happen right away after the switch so I don't known if there is a causal connection. 3. Finally, my HR23 is only connected from the dish to one tuner. The other tuner is dormant. I tried switching the dish input to the other tuner only to get a message that the receiver wasn't properly connected to the dish. Don't know if that's a possible factor. I still don't have 0x66a or 0x66b or whatever is the latest now. Maybe it will help. Thanks!