My 58" C550 has the same purple banding problem. At first it didn't happen too often so I ignored it. Bad idea. Now it does this all the time on white backgrounds. The set is just under 3 years old.
Today, I called Samsung and they refused to offer any help with a repair since it is out of warranty. OK, I get it now. How silly of me to expect your TV to last 3 years. The rep from Executive Customer Relations put the final nail in the Samsung coffin when she told me that their engineers are "not aware" of this problem. Apparently, they must not read the customer forums on their own web site.
Dear Samsung: Sorry, but I won't be buying another one of your disposable TVs. Oh, and one more thing. I am "not aware" of any reason to purchase any of your other products either.
With all of the people having this problem, it shouldn't be too long before someone files a class action suit.