[Jack]

Larry is far more expert than I am, for sure. I would follow his recommendations first.
When you lower the slide by hand, are you doing it slowly? A tight gun might hang up a bit doing it that way. If the slide hangs up when you thumb the slide release and let the slide go forward on it's own, that's a problem.
You mentioned that the hang up occurs when the pistol is level or pointed down, but not when it's vertical. That would make me suspect the link might be binding.
The only other thing I'd suggest is to check the lug recesses in the top of the slide- if they get some crud in them, that can hang up a tight gun, too.