Darkened - compared to what?
Picture settings for external sources almost always need adjusting. If you're using a TV Game mode picture setting pre-set the chance of it being useable, if my experience is anything to go by, is nil.
What type of lines?
Any other external equipment attached or unshielded cables nearby?
I know there might be good reason for you only using Composite but S-Video (US), RGB SCART (UK/EU/AUS) or best of all Component will give you noticeably better picture quality. Of course if the Composite cable is the source of the problem, as it could be, you'll need to replace it anyway.