Many more hours spent troubleshooting & more data available.
A complication is it sometimes takes up to an hour to reproduce the problem! I open the INFO screen which displays the picture smaller & moves it to upper right & the Cox DVR software displays program info & then I scroll up & down the channels & sometimes after 10 channels the 1/3 black screen appears & sometimes it takes 350 channels b4 it occurs. Again, the issue has only occurred after doing something with the Cox DVR menus, never while just watching TV or BD/DVD. All HDMI cables are 8-10 feet.
Long phone conversations with tech support; Cox DVR, Panasonic TV, Pioneer AVR. None of them think it is Their equipment.
I was able to connect a 2nd small LCD SDTV using 75 Ohm coax to the cable out port of my DVR while leaving the HDMI connection to the big Plasma alone. So, I got identical pictures & controled both TVs simultaneously. Using the Info menu & scrolling channels. I was able to reproduce the problem several times along with a new problem that happened even less frequently. Always both problems occurred only on the large plasma while the small LCD TV picture was just fine. The new, rarely seen problem was instead of a picture, the TV displayed colored noise Sometimes colors filling entire screen & other times colored the top 1/2 with the bottom black but not black as in no illumination of the pixels (like I see with the 1/3 black issue). Sometimes within the black there were 2 vertical grey rectangles. More like black snow in part of the picture. Once a black, thick pillar box sized rectangle on the left with a horizontal black rectangle on part of the bottom & the rest a purple color. This rare colors' problem seems to be a signal issue which may mean the 1/3 right, black problem is too. Another new fact is I no longer need to turn off & then on ANYTHING in order to fix the problem. If I just continue scrolling channels after 5 or 10 it will fix. I have gone back & forth with some channels exhibiting the 1/3 black & others remaining just fine. Again, this is always on the big plasma while the small LCD never had any problem even though both hooked up to the DVR at the same time. I checked & the channels with vs. w/o the problem are not all 1080i vs. 720p. So, that cannot be an issue.
I concluded (in error?) that this means the DVR is not the problem. That leaves; TV, cable or AVR. However, the LCD TV did not use HDMI nor the AVR.
Next, I bypassed the AVR, I connected the TV direct to the DVR using the HDMI cable that I suspected of being bad. It normally goes DVR - AVR. This cable is very THIN, it is still 1.3 HDMI but I paid $2.00 on Amazon for it believing that expensive HDMI cables are a waste since they either work or do not work. My other two HDMI cables are from monoprice & 24 AWG, CL-2 rated, thick & stiff. I just ordered a new 24 AWG from monoprice to replace the thin, cheap one just in case.
So, using the thin cable that was DVR-AVR I went direct DVR-TV. I scrolled channels for HOURS & was unable to reproduce the problem. Eureka! I thought. It is either the cable or the AVR.
Why did I still think the now working, thin cable may be bad? I read that when you route an HDMI cable through an AVR you need better quality cables (24 AWG) vs. if you just run direct from source to TV. The Pioneer AVR tech guy agreed with that. So, I thought maybe my thin, cheap HDMI cable may work DVR-TV but going through AVR I have the problem.
Next test was to swap the AVR inputs. So the bad' HDMI cable that normally went to TV/DVR input on the back of the AVR was switched to the BD input & vice versa. This was suggested by Pioneer as the best way to test for a bad AVR. The same 1/3 black screen problem reoccurred! Even when using the BD input. So, unless the AVR has TWO bad inputs the trouble is probably NOT with the AVR.
Finally, I swapped HDMI cables with the thin bad one BD-AVR & the good (24 AWG) ones DVR-AVR & AVR-TV.
With this last test, I really expected the problem to be gone & then I'd swap out the bad', thin one with the new 24 AWG one when it arrived. I would have bet that would happen.
Nope! Problem reoccured!
I am almost out of ideas. Pioneer suggested that a weak signal from the DVR may work OK until it is routed through two cables & the AVR & he thinks it is most likely the DVR despite the DVR playing without problem when connected via 75 ohm coax to a different TV.
So, I might try to get a new Cox DVR, losing all my saved programming. If that fixes it --great, if not, I lost all that programming & still have no solution.
Any ideas welcome.