I own many samsung products, i have spent to the tune of 3000 USD on them & a 51" Plasma TV is one of them, within 4 months of owning the product; lines started appearing in the panel, like the one you have described here and in 15 months the panel is literally disintegrating line by line on a daily basis. I have even started a blog to describe their horrible quality practices right here :

http://poorqualitysamsung.blogspot.in/

Please visit the blog it has some pictures too.