TV displays vertical lines when using a Mega Drive SCART
The common cause of the vertical lines occurs when using a regular "sync over composite video" RGB SCART cable. A composite sync or CSYNC RGB SCART cable will significantly reduce the vertical lines which are often referred to as "jail-bars", however, you will loose the ability to transmit composite video, which isn't such a bad thing.
All our Sega Mega Drive and Master System cables are wired for CSYNC however if this doesn't fully resolve the jail bar issue you may want to consider an RGB bypass modification on your console to completely remove the vertical lines.