Yeah, it slipped my mind to point that out. I scrubbed down the contacts of the games with q-tips and isopropyl like I do with all my games, and since we opened up the system, we cleaned not only the cartridge slot but the whole board (though the board was pretty clean to begin with). This stuff is about as clean as it can get now. If it were a matter of cleanliness, I would expect the 2600 games to be just as finicky about working, but they're pretty reliable now.

I've read that different chips are responsible for playing 2600 and 7800 games and that, in cases where 2600 games play but not 7800 games, it could be a bad MARIA chip, which is responsible for playing 7800 games. But since I got a 7800 game to play one single time, I don't know if that's confirmation that the MARIA chip is fine. The 7800 I found is apparently among the earliest models, so I'm also lucky in that many of the chips are socketed rather than soldered directly to the board. It's easy to pop them off and back on, so we carefully looked for any signs of damage. But there's just nothing jumping out as the culprit.

I still have some more 7800 games I need to clean, so I suppose I should see if there's any game among the bunch that can work better than the rest.