All good to know. My monitor chassis isn't grounded to anything for sure. And dang, walking!? Glad to hear you're better now.
I've done a few things now. I connected the VGA from the computer directly to the monitor by twisting the syncs together like you mentioned. No JPAC. Here's some comparisons between raw VGA and JPAC. The first image is raw VGA, the second is JPAC.
Blegh, Bison's gums are a mess. Based on this, it looks like the computer is outputting those white ringing spots even without the JPAC. It's especially noticeable around the edges of M.Bison's cape. The JPAC is just amplifying the ringing. So I made a low pass filter at 220 microfarads and 75 ohms and I get this:
The image just turns blue? You can faintly see the icons on the computer desktop. I made sure there wasn't any wire issues with the red or green. As I removed each capacitor, it restores that color. If I remove all 3 caps, the image looks normal. I also tried higher and lower capacitances. The higher capacitance didn't change anything. But if I went lower, the image would become slightly less blue. I went down to 0.1 microfarads and it looked like someone smeared the screen like a hot knife on buttered toast. I also tried flipping the capacitors +/-, but that didn't change anything.
I connected the input red, green, and blue to the left of those resistors, and the output RGB to the right of the resistors. I cut a very heavily shielded VGA cable in half, and spliced this low pass filter into it. All the capacitors were grounded to the main ground. I tried video ground too, but nothing changed. (ignore that one flipped capacitor in the photo)
I'm thinking I need to "clean" the signal from the computer with some external device, I'm just not sure what yet. And I'm sure some might think that the image looks fine, I'm just being picky. And you're right. But the Neo Geo outputs a fantastic signal with no ringing so I want to get that from the computer as well!
Thanks again for the help so far, will continue to figure this out.