There have been multiple threads on issues affecting picture quality. I would like to offer another reminder to eliminate HDMI cables as the first troubleshooting step. Today our picture started cutting in and out, with lengthy intermittent black screens. It came out of nowhere for no apparent reason. I have everything daisy-chained through a complicated series of switches and devices so diagnosing was a bitch. I did think to check the connection fairly soon and in the process I noticed the HDMI cable into the TV was not fully inserted. I replugged it but that did not solve the problem. After painfully threading my way through the entire daisy chain, I eventually arrived back at the cable. Thanks to our dear departed Dick Smith, I have abundant replacements and that did, indeed, solve the problem. Yet the faulty cable looks perfectly okay. No sign of any kind of physical wear or damage. Also, it normally never gets unplugged. That is what all the HDMI switches are for. 

The only thing I can think is that one of our cats, who sometimes go behind the TV, dislocated the cable and this somehow damaged the connector. These cables can be surprisingly vulnerable so if you ever experience a problem like this, my advice (along with many others) is to check the cable first.