It's the TV... Something that many Dreamcast owners have experienced in the past 10 years.
Personally, I prefer using A/V to Scart because the image quality is better. My cable type has the option of both, which I connect a 6 metre AV cable (avoiding the scart connection).
I would personally suggest this:
http://cgi.ebay.co.uk/Sega-Dreamcast...item2eb3abcba4
Bookmarks