JPAC and some video modes are shaky

[Ummon]

I did some searching and couldn't find anything on this. I'm using a JPAC with my candy, the monitor is standard res, and there are times when some modes will be sorta shaky. 320x240 is always stable, regardless of PC/video card combination used. However 336x240/most other modes  sometimes aren't. Like tonight I was working on it, and those modes were always shaky. Is it my (really old model) JPAC?

[AndyWarne]

The J-PAC passes the video straight through. In fact on your very old one its literally right through as there is no video amplifier. It does generate composite sync though but this is unlikely to be a problem.
The video source is the most important issue.
Also make sure the composite sync is connected to the negative composite sync input on the monitor.
Andy

[Ummon]

That's what I thought. I didn't change anything with the harness, and there was no similar issue with the game board that came with it. The issue has been intermittent, though, and lately has been doing it on many modes very consistently, so maybe it's something with the monitor?

[AndyWarne]

Have you tried adjusting the V and H hold? Note on W-G older monitors, the H Hold is on the main board not on the small control board.
Andy

[Ummon]

I thought of this, and I suppose it could be the issue, or at least address the symptom(s), but was hoping it wouldn't, because though it's a standard res monitor, that still allows me to run horizontal games of different resolutions. I'll see.

[Ummon]

Okay, that's not it. It's actually a vertical jumping up and down, but turning the Vhold (which is on the front panel inside the CP) either way makes it roll, so I'm figuring it's where it's supposed to be.

[Ummon]

I'd like to reiterate that the Tetris board (game resolution is 336x240) is stable, yet when I run 336x240 from my PC, the image shakes. Anything above that is stable. I'm using soft15khz, incidentally, but remind that the shaking phenomenon has increased in frequency and scope.