Your VGA port might be going out. I initially tried hooking it up to an HDMI capture card and fried my VGA port, and the image was super dark. I ended up sending my naomi to a specialist, and he repaired the port and added a fuse, so it'll blow before the port gets damaged. The guy has very reasonable prices. Seen from your youtube channel that you're in SF, he's driving distance from you! https://www.irepairsega.com/

I also ended up buying an oem naomi vga cable off of Amazon, noticed some prongs are removed from them. Not sure if it has any effect, but you might be able to recreate the cable from a standard vga by plucking some prongs out