Here's a quick test for you. It'll isolate a possible tuner issue. 1, ensure there are no upcoming recordings.
2, set 3 recordings going on 3 HD channels.
3, press Home, go to help & settings, then system info, diagnostics.
4, now scroll down and you will "channel sources" and just underneath Tuner 0. Make a note of the channel it is recording.
5, Scroll down and do the same for Tuner 1 and 2
6, Stop the recordings and play them back. If you find a pixilated recording look and see which channel you were recording and what tuner was in use.
If you do find a poor recording, try again recording different HD channels. Now see if there is any commonality of bad recordings and the tuner used. I've found you'll only get it occasionally when all 3 tuners are in use, but it may give you something definitive to push to VM for a replacement Tivo due to a failed tuner.