Problem with V-hold on some resolutions

[pacman]

OK first of all here is my config :

JPAC + Arcade VGA on Hantarex MTC 9110 horizontal monitor (capkitted)
Mame Windows + INI's generated with AVRES tool (cannot use Mamewah tool as it crashes on launch on my XP) 
A PC with all the stuff in it
Front end MAMEWAH in 640x480

I am experiencing many difficulty in keeping the V-sync stable when changing to certain resolutions. To sum up here is what I did :

I tuned the V-synch to have a stable image in MAMEWAH with 640x480. At that stage if I launch a vertical game (no matter if the res is the orginal one or with Hardware Stretch) then it works fine without rolling or shakings. This is not the case when launching Horizontal games ( especially those using very low resolutions)...very hard to have a stable image, it rolls quite fast and so I have to adjust the V-sync pot again and consequently I just loose the V-sync for MAMEWAH and Vertical games. This is making me crazy!!!!! Just to let you know that INI have been generated with the 'only 60 hz' option enabled.  Please help 

[AlanS17]

Are we talking about a slow scrolling, a fast scrolling, or a giant mess on the screen? Is it something you could capture with a camera or a screen capture??

Hmmm... and you're sure the vsync going into your monitor is actually wired up?

I had a problem like that on a Darkstalkers pcb and the problem was that I was getting signal to the hsync but not the vsync on the chassis.

How many wires are leading into the chassis? Are the all secure?

[pacman]

I do not have a camera available for the moment but what can I say is that the rolling is not very fast (no mess on screen), let's say the screen will make a full roll from up to down in about 2 seconds.

Do you think this can be a V-sync wiring issue? I am not sure because as explained all V games and stretched games ar OK and stable until I adjust the V pot to have those bloody H games being stable. Also as I said not ALL the H games rolls, only the ones using very low resolutions (eg 256x224). H & V sync wires are connected together to the composite sync pin of the Jamma.

[pacman]

Somebody with a suggestion ??