@lovablechevy - Yes, I've already seen these guides. Also, I was told by Sega-16 member OoXxXoO(see here, that's the thread at Sega-16 where I keep track of all progress on my Neptune:
http://www.sega-16.com/forum/showthread ... 299&page=3 - first post on the page) that the plastic piece sitting between the cartridge slot and the bottom RF shield may be part of the problem. I'm also doing things a little differently. Like Remyar, I've kept the metal rings over the ribbon cables, and unlike the other Neptunes I've seen, I'm keeping the metal plating over the cartridge slot.
@remyar - (English) - Aluminum foil covered in plastic, you say? I guess my theory of "cross talk" may be correct. It's weird, though, because I use IDE ribbon cable, which is supposed to be perfect for a job like this. You used IDE ribbon cable, right? Should I maybe replace them with 22AWG wire?
(French) - Du papier aluminium recouvert de film plastique, tu dis? Je pense que ma theorie de "cross-talk"(je ne sais pas comment dire ca en francais) est belle et bien vraie. Ca me parrait bizzare, pourtant, car j'utilise du cable IDE, qui sont supposes d'etre parfait pour ce travail. T'as utilise ces cables, pas vrai? Devrais-je les remplacer par du cable 22AWG?