I have 2 Tivo Premieres that are having exactly the same problem as what is listed in this thread but mine could be on either tuner.
I have 2 Tivo Premiere units for almost 3 years and one regular Comcast HD STB for small TV in the kitchen. Tivo units worked flawless for at least 2 years.
At some point in 2013 I started getting recordings that did not occur and checking history on Tivo it says no signal. A few times I have caught it in the act and nothing recording and the channel says Not Authorized or there is a blank screen. These are all on HD versions of CBS, NBC and ABC which are 611, 612 and 613 in my area no special channels. Sometimes this happens when I am channel surfing but if I move back and forth between channels it will sometimes tune. Once tuned it seems to stay unless I change channels. Some days it does not do this at all.
This occurs randomly on both Tivos that are identical but not always the same channel on both. If I setup the same recording on both, I can get it on one but not the other. When it occurs I can go to the Comcast setup box and it seems to work no problem.
What I have tried:
1. Checked all my physical cable and signal multiple times. Seems like this would not be a issue since the STB always seems to work.
2. Have Comcast support check M-Card pairing multiple times over the last year and refresh signal multiple times. Last refresh wiped out everything and I had to re-setup modem and register all over from the ground up.
3. Worked with TIVO support multiple times and everything points back to physical cable issues, M-Card pairing or subscription problems.
4. Last weekend we had a cable cut in the yard. The cable run had been replace 5 years ago and whoever buried the cable ran it over the top of a sprinkler. Repaired the sprinkler and cut the cable.
I worked with the tech while he was here and repaired the cut and checked cable and signal at all locations. After describing the problems he suggested some things and he removed my Motorola AMP that has been working for 10 years and installed a Unity AMP from Comcast. The signal and SNR are perfect on all ports. The tech was excellent BTW and did a great job. While he did not really find anything wrong other than the cut he made changes and checked signal everywhere.
Needless to say the next day after the tech was here I started getting the same channel issues and dropped recordings.
Last discussion with Tivo today the tech suggested replacing both M-Cards or both Tivos. If I really believed both units failed identically I would do it but it beyond unlikely. I have 30 days left on my warranty so I need to decide something.
I have a case manager working with me from Comcast as well but so far no luck.
If you read this far, thanks for listening. If you have any suggestions that I have not tried I would appreciate them.