S video (from vga box) to hdmi stretch(?) marks/ ghosting on certain parts

[Mechasonik]

When using the vga box s video and an s video t hdmi converter, when the dreamcast boots up i can see black stretch marks from the word dreamcast and then when the sega logo shows up I see like ghosting over it and sometimes the screen flickers. I don’t know why this happens but when I try the same with the yellow, white and red cables (from the hdmi converter) it shows the screen without problems. I could upload some pictures later but I don’t know if they’ll show what I mean. If anyone knows what this is or why is it happening I would be very grateful!

Thanks!