I experienced the same problem with a PN50C6500 Samsung Plasma TV that I purchased in 2010 as well. There are thousands of complaints all over the internet where people are developing horizontal lines on their 50" Plasma screens that were purchased in 2010. The model numbers all vary but they are all 50" screens and were all manufactured/purchased in 2010. All posts say the same. contacted Samsung and was told Warranty is expired and that Samsung cant help. I was even told that Samsung had received no other complaints about this issue (Yeah OK).

I have since given up on trying to get this TV fixed. I have purchased 3 TVs since (2 LGs and 1 insignia). Still have the Samsung but the kids use it for a gaming tv. They say that they notice the lines less when playing games.
I just hope that the people that have posted that they will never purchase a Samsung product again actually mean it and that word gets out that Samsung hides behind their warranty and does not stand behind their products.