Xbox color messed up during movies?

Whenever i play a movie on my xbox 360 (from netflix or a disc) the color is always messed up. It will get really dark for a while, go back to normal, then quickly go back to being dark. My old xbox (1st generation) did this also. somebody told me it was a safety feature? how can i make it stop doing this? using regular AV cord directly into TV. It is a very old TV but it only does this when i play movies on the xbox.