TWR, something else you might try is removing the buffer and running a 16 lb recoil spring. Base on the limited info, it seems your gun may be over sensitive to anything that restricts the slide from fully retracting and allowing enough time for that last round to come up all the way while the mag spring isat it's weakest state. I might be wrong, but its a cheap experiment. If it runs that way, there is no reason not to leave it thus.
