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Hi all,
So for those who have been following my progress (and helping me a lot on the way) you will probably know I started with a spare 505 and spare 527 RAM boards. I built a backplane and Klyball data splitter and got the boards working as a serial system with floppy access. I then built a 540 board, and with a bit of modifying I got an early revision of Dave's Unified/Universal Keyboards working. So now I am really close to my end goal, however I'm having some annoying issues with the video sync...
The 540 does display video (so thats good), and I have tried the setup on various CRT monitors with varying success. With one monitor, the best I can get is to de-tune the vertical hold, and I can get the image somewhat stable, but eventually because the vertical hold is right on the edge, the screen will start to roll. If I tune it slightly back, then the image can be in the right place, but jumps and jitters a lot so also not ideal.
I am in NZ which is PAL land, and the 540 board is running a 12mhz Xtal. I have (I hope) built it with all the color circuitry installed, but actually ended up temporarily removing the color crystal for trouble shooting. I have played around a lot with all the adjustment potentiometers too. I have a variety of other US systems (including a C4 and a C2) and I've never really had any issues with getting a clear stable picture out of them on my monitors.

I have attached a video showing what it does on a monitor without all the mucking around with vertical hold. (This monitor actually has no vertical hold adjustment).

I was wondering if anyone could help or if they have any insights into where I could look.
Thanks again.

Newbie here: What is a "Klyball data splitter"?

Here you go:
viewtopic.php?f=3&t=220&hilit=d13
Allows you to use drives that are not original MPI.
MPI drives had the data splitter built in.

Thank you :-}.

And for those interested, here is what the build looks like so far.
I designed and printed a slotted 3d bracket to hold all the board in place so they aren't flopping around. It's also a convenient place to put a reset switch, video out and a small breakout board that makes removing and attaching a floppy cable much easier as access to the data separator is tight.

the 540 board is running a 12mhz Xtal.

The color 540B boards built by OSI used a crystal frequency of 12.083915 MHz. That's only about 0.7% higher, but if your board is right on the hairy edge, maybe that would make enough of a difference? I know there are places that sell crystals with custom-ordered frequencies. (The optimal [to the nearest 1 Hz] crystal frequency to produce a horizontal frequency matching the NTSC spec would actually be 12.083916 MHz,) Before ordering a new crystal, maybe try swapping your 12 MHz crystal with the one on the C4P that works well, to see if it makes a difference?

Looking at video of the monitor you provided, it looks for me like a vertical hold issue of the monitor.
Check if you have V Hold controls at the back of the monitor or inside and try to adjust.
If this does not change anything, you should have a look to the vertical sync level at the video output jack (monitor connected). If the signal level gets to small, vertical hold effects will show up.

I have played extensively with vertical hold on multiple monitors (see original post), but with little success.
I will order a 12.096MHz Xtal from mouser and see if that improves things. It's still not perfect, but closer.
Don't want to pull the C4 apart as it always takes a lot of mucking around to get it working again after disassembly (I suspect some dodgy contacts to the backplane).

Ok...good news. Had some time to look into the sync issue last night.
I discovered that if I touched the traces around C17, C18 and C19 the constant scrolling would stop. Looking at connected components I focused in on the Aliexpress special 74LS123 at U4A. Replacing that with another branded IC fixed the problem. Now I a have stable (if not a bit noisy) image.

Next I will try another monitor and see if noise persists, and if color works?

Can I ask what power you're using for this machine?

The reason I ask is that the interference looks really like some I experienced with an Atari 600XL, and the issue turned out to be due to a Chineseum PSU I received with my OSSC.

Simply changing to a different PSU resolved the problem for me.