i have a problem, i am using normally 1024x768 over hdmi out for my 4:3 Monitor. In this mode i get with the core no picture, Audio is working fine.
Also tested 800x600, 640x480. DVI on/off. Other Monitor. No Picture in 4:3 Modes.
720p, 1080p working fine. VGA out also.
The refresh rate of neogeo is 59.1856Hz. If you have vsync_adjust=1 or vsync_adjust=2 then your display probably just has trouble with that. Other cores like Genesis, Nes, Snes, Tg16 output refresh rates closer to standard 59.94 or 60hz rates.
Interestingly, AES neo geos had a different master clock (24.167829MHz) and output something like 59.6hz. Probably for display compaitibilty...
But as it stands, the neogoe core is just going to work with few displays than the other cores that output near 60hz.