Pal/NTSC issue perhaps? Using scart, I would assume you're in Europe/Pal area though. And NTSC on a PAL display usually involves losing the colors, but some devices might have trouble locking onto the vsync with a 10 hz difference. (My old 1702 monitor did the same thing with PAL outputs, but can be stabilized with vhold/vsize knobs, I'm in US (NTSC))
