Connected my Atari, finally, to my TV set-up. It's connected the TV in coax port of my Genesis switchbox, that switchbox is connected to my converter (modern RCA -> COAX, very useful), which is connected to my VHS, which is connected to the TV. It's a horrible method, yet it works.
Anyway, I can play some games like Combat and Missile Command and even Mario Bros. fine on it, but once I choose something like Swordquest and move my character, the screen flickers like mad. I thought it might have been my channel signal selection, yet it changed nothing. My TV has no option on flickering, though, so I can't really do much. The other option was that it's a PAL cart, but I tried my duplicate and the same thing happened.
If anyone wants, I can record myself trying it just so there's some idea on what's going on, but I think that data might be enough. Running on a Darth Vader.
EDIT: Tried every channel combo possible, still nothing. I've tried a few puzzles like the nuclear waterfall, which flickers if you DON'T move, and the run-across-a-highway thing, which flickers whenever it wants.