On my kids XBOX 360, the cable has a switch for regular TV and high def TV and you use different AV plugs depending on which. I would say to make sure the switch is fully in the correct position for the TV you have it hooked up to and double check that you are using the correct AV plugs.
that is one of the main problems with the system. you will have to replace the cable to fix it. they were made poorly at the factory. oh and beware the rings of death!!!!! 3 red lights that will pop up and stop the system. good luck!!!