I had this exact problem. It ended up being a bad video cable between my pre-amp and my dvd player. Yeah, I know... how could that have anything to do with it? Seriously though, that was it.

Disconnect everything from your stereo. And reconnect one cable at a time... turn your components on, and send a signal through each as you turn it on. You will probably find that there is a combination of 2 powered on units causing the problem.

Another way, to be quicker, would be to turn off one component at a time until the problem goes away.

You could also have a ground loop... either way, it is easily solve-able given time.