PAL Dreamcast at full screen flickering issue

[MLConian]

I recently bought a PAL Dreamcast and hooking it up to my CRT, I noticed a strange issue. Whenever a game forces full-screen mode, the top few centimeters of the screen will flicker. It’s most notable on blues. There is no flickering present on my flatscreen TVs, but the only one of those 2 that is permanently hooked up introduces so much lag that anything other than a turn based RPG becomes unplayable (that, and my wife doesn’t take kindly to me hooking up the Dreamcast in the living room when I’ve already claimed an entire room for my retro gaming).
In 50Hz mode, most games output a somewhat smaller screen and there is no flickering at all, but my import NTSC games all run in 60Hz full-screen with this issue and, most notably, the PAL version of Skies of Arcadia runs in full screen as well and has this flickering band at the top of the screen. At first I thought it was the TV giving up the ghost, as it’s an ageing Panasonic CRT, but all my other consoles (all through composite) work fine on it, and it doesn’t matter if the content runs in 50Hz or 60Hz, or if it’s full-screen or not: there’s simply no flickering.
I figured it might have something to do with the cables, so I ordered the Retro-bit S-Video cables and an S-Video SCART adapter (my TV doesn’t have an S-Video in), hoping the problem would be gone by taking the video signal from different pins on the motherboard. The cables came in yesterday and perform exactly the same as the stock cables on composite, the S-Video adapter just arrived and sadly when I plugged that into my TV the output got even worse. Not only is the flickering still there, new problems are introduced using S-Video (halos and shimmering). Under normal circumstances, I would’ve just shrugged it off and got a VGA cable (which is actually on its way, but delayed), but as most of you probably know, Skies of Arcadia doesn’t support VGA – and that game is the sole reason I wanted a Dreamcast (already have Legends on Gamecube). I’m looking at buying an OSSC Pro and a Koryuu when the Pro comes out (retro game collecting in Europe isn’t much fun without a CRT, I hate being stuck with 480i/576i and no proper way to deinterlace for modern displays), but as the S-Video “trick” didn’t solve my problem, I’ll still end up with a flickering Skies of Arcadia playthrough
Does anyone know what causes this problem, and how I could solve it? I’m not above buying a second Dreamcast (that’ll give me one to mod), but then I’d still like to know whether it’ll have this issue as well and how I could recognize a Dreamcast that doesn’t have this problem.

[BuckoA51]

If the TV has SCART, how come you’re not using RGB?

[MLConian]

I was thinking of getting RGB cables, but then I read some comments on Reddit where people used the Packapunch RGB cables on a PAL Dreamcast and still have the same issue. Other than that, RGB SCART cables are nearly impossible to find in the Netherlands.
I did manage to find another Dreamcast, and it actually has the very same problem. I was able to capture a video of Sonic Adventure 2 NTSC-U running on it. The same flickering band pops up on 60Hz PAL games, or anything that runs in full screen (I’m not sure Skies of Arcadia PAL actually runs at 60Hz, but it’s definitely full screen), but it was easiest to capture on Sonic Adventure 2.

I’m starting to think this is caused by the CSYNC signal if the video processor runs in PAL mode, because it’s supposedly gone on VGA, which uses VSYNC and HSYNC on separate pins and ignores the PAL/NTSC processing (according to the Dreamcast service manual I found) – but I have no means/skills to actually verify that. I do know, however, that the RGB cables still use the CSYNC pin (pin 10).

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