Mamewah with soft15khz / 15khz monitor

[ac3]

I've used Mamewah in the past on a 31k monitor and it worked great but now using it on a 15khz monitor I am having problems. It looks good when set to 640 x 480 but flickers, I have tried lower desktop and mamewah resolutions but then the main menu and sub menus don't fill the screen properly or flicker (or both). Can anyone recommend a desktop / mamewah resolution that will fill the screen and not flicker and wont require me to constantly keep resizing the image with the monitor controls. I don't know if I am changing the menu resolutions correctly but I've been doing it in the layout editor. 

Any help is very much appreciated Ace`.