Lets see some pictures of that Slug 3. Was it just the cart? That is such an absurdly great find, I can't even fathom how something like that would end up in the garbage.
Definately buy these:Fatal Fury Special ($20)
Samurai Shodown/Spirits II ($20)
+1 on the King of the Monsters games
Consider buying these:Samurai Spirits III ($60-ish)
Samurai Spirits IV ($150 -- the US version is pretty rare and commands a higher price FYI)
Sengoku 3 ($180, one of the best beat 'em ups ever made, fabulous fighting system)
Last Resort ($150 or under, one of the best R-Type clones ever, probably my favorite Neo shooter)
Aero Fighters 2 (not sure on the price but I think it's under $100)
Never buy:Metal Slug 1 (costs $1000 minimum and you have to be an expert to spot a counterfeit)
Sell:Riding Hero
Ghost Pilots
(both games are seriously TERRIBLE)
Theres a ton of fun fighting games in the $100-$300 range if you're into that sort of thing. Some of my favorite fighters are KoF 98 and 2002, Art of Fighting 3, Matrimelee, Garou MOTW (under $400 for a JP copy), and probably others I'm forgetting.
Also consider selling all your stuff (or maybe just the Metal Slug 3 if it's complete) and buying a consolized MVS board, an MVS board and a supergun, or go all the way and get a cab with a nice low-res monitor.
Oh, and when you notice the horrible dot crawl (i.e. rainbow distortions in the graphics): there's nothing wrong with your system, it just has terrible composite output.