I'm not going to make any claims about being able to diagnose your problem without ever having seen your gun, but I'm a test engineer, and I spend all day designing tests to find problems. Maybe I can suggest some things to look at....
First, we know that the first magazine reliably reproduces the problem, which is that every two or three rounds a round fails to feed, and at the same moment your mag appears to partly eject, but not fully until you press the mag release button. Second, we know that the second magazine works flawlessly. We also know that a changed spring in the first magazine didn't solve the problem.
We can deduce that the rest of the pistol is not the sole cause of the problem because it too works fine when using the second magazine. The problem probably lies in a mechanical difference between the first mag and the second, and is probably in the first magazine itself.
I would carefully compare the good mag to the faulty one, and l would focus on searching for any dents or slight differences in alignment between the two. I would also look at wear on the magazine catch slot, but that seems less likely a cause to me. My hunch is that one or both feed lips on that mag have been bent slightly inwards, thus forcing cartridges into a slightly lower alignment relative to the slide and the feed ramp. Experiment with some round-nosed snap caps to see if they're hanging up on the feed ramp more with that bad mag. I'm still not clear on how the mag is pushed downwards during the jam, but my mind's eye can image the round catching on the underside of the top of the breach, with the slide pressing the tail of the round downward into the magazine, essentially spring-loading it downward against the magazine catch. (I don't really know if this is what's happening, so correct me if I'm imagining incorrectly.)
Really take your time, and use your pondering chair as you look it over. Good luck, and let us know what you discover.
Scott
