Zenith tv problem..

I have a Zenith TV thats about 6 years old and I've always had the same problem with it. The screen is "stretched" out , for instance I can't see the news-ticker on the bottom of the screen on CNN or my volume bar anymore. My friend has the same TV and has the same problem with it. I had it repaired once and it fixed the problem, but a few months later it seemed to slowly start stretching out again.

I have no idea of how to fix this, maybe its a simple problem but I can't seem to find anyone else who's had this problem with any TV before, any help is much appreciated, thanks in advance.