Originally Posted by
savageone
It's definitely your CDX system, I've yet to play a game on my CDX that did not work where it would on another Genesis/Sega CD, this includes the 3-4 supposedly incompatible CD games (though I honestly didn't play them 100% through). The only games that should have a problem are the earliest unlicensed games, but those wont work on anything but the earliest model 1 builds that have no copyright screen iirc.
Maybe you've done this by now, but since you've already tried cleaning the slot you could try getting a flashlight and shining it down and looking for any pins that look bent or corroded. They're a pain to see on the CDX but my best guess is you may have 1 or 2 bad pins hence some games work and other do not.
The Sonic CD audio might be another clue though, another test I would try more CD based games. Try the Sega CD version of Flashback for instance.. Who knows? If there really is some internal problem there is a chance that the problem you are having with the cart version will be replicated on the CD version.
Past that you'd have to put on a determined face and open it up and look for anything out of the ordinary. CDX systems are a royal pain internally. :\