Bought first Dreamcast but problem with picture

I bought a Dreamcast over eBay but notice distortion with the picture on my tv screen.

It seems to be okay for darker games but on ones with bright colours such as Crazy Taxi the picture becomes blurred and distorted.

I tried another tv and while the picture was sharper there were five/six horizontal lines across the top of the screen.

I'm using an RF unit to connect. Is this a normal thing for the Dreamcast or is the RF unit or the Dreamcast itself??

Please hep! Thanks for any help

Sounds like it could be bad cords or loose wiring in the system.

Most likely a bad rf lead

ive had a few crap ones, my advice by a new un or a scart cable :)

Yeah cause ruddy RF units have a habbit of breakin, mine does from time to time, but if i hit it against somethin it works again. yeah 8)

scart gives a massive picture improvement too. a very noticable difference.. rf looks so washed out after using a scart.