I've seen this issue before and its fixable...its usually one of the bottom line of ribbon cables that have either gone bad from overheating so much or simply has come loose from vibration. Time and time again it seems to be loose from vibrations over a few years of spinning fans.
Here is what it looks like, like i said..most of the time (at least from one of my own experiences and plus reading other people's experiences at "other" AV forums on the net...is most of the time fixable..but some do go bad from worn tracks on the ribbon cable from vibrating because of the fans. I had a G10 one go bad where i fixed it...but about a year later something else went wrong.
Anyways below are a few pic's of what im talking about.....these two pictures below are from two different TV's so the color of these ribbon cables will look different depending on tv,tech,year,etc and keep in mind these ribbon cables are lined all the way across the bottom of the tv from left to right..the bottom pic is about where they are located on most plasmas...where the black line is, is where you need to look at inside your tv.
PS I really didnt read the entire post here so if you already got rid of it or fixed the tv...forgive me then, i am just sharing this info in case someone wants to give it a whirl with nothing to lose. I have a G10 in the back room that i've thought about bring back out to tinker with and see if i can fix its few issues....currently have a less than one year old UT55 Panasonic which is rock'n hard with super nice clean crisp IQ...best bang for the buck tv i ever bought..even better than my better G10 model i had before this UT55 inch....scored a home run when i found this current UT55 on Amazon for 600 bucks which included shipping!! 55 inch of goodness!
EDIT: It just hit me, the bottom pic is actually a sideways pic of the back of a tv's right or left end...not bottom...but the bottom of plasma tv's has these ribbon cables too that effect these black bars or scrambled color bars...you get what i mean geesh lol
The bottom of the pic has a bunch of those brown ribbon cables...this is what im talking about...some tv's have the steel brace over them like the top pic does..and some dont..but where ever you have the black or discolored lines on your tv..is exactly where you should be looking where these brown ribbon cables show..if its on the far right of your screen...look at back corresponding position of one of those ribbon cables...wiggles it a bit to see if it issue flickers off..that is if you want to have the tv on wlhile doing this but be careful and whear gloves or do it with tv off....what ever, im outta here...GL