As far as the slide locking back with ammo in the mag....I assume you're right handed? It happens to me once and a while and always turns out that my support thumb (left hand) occasionally bumps the slide release in recoil.
I used to do that too, especially with a friends pistol w/an extended slide release. The regular GI part,,no problem.
A couple of other mechanical things to look at when it all of a sudden starts happening are the ammo.,,
The edge of the bullet is supposed to clear the slide stop inside the mag. But some will just barely be able to touch it, especially in recoil when the rounds in the mag are as far forward as possible.
Even w/ball ammo of the exact same weight, the bullet shape can be ever so slightly different between mfg'r. OAL can be different by a few .000" and cause a problem when mixed w/ bullet shape and the right magazine.
Sometimes just switching magazines will correct the problem. They are not all created equal, even if made by the same 'Maker'.
The slide stop/safety spring & plunger assembly can also allow the slide stop to engage while live rounds are still in the mag during firing.
If the spring is a bit weak, and/or the recoil is all of a sudden more than experienced before (as in lightening the springs), the plunger will in an instant loose it's proper detent engagement on the slide stop and can occassionally
catch the slide on return and lock it.
Check for a loose slide stop spring 'tube' also. It's rivited (or at least used to be on 1911's) to the frame and can come loose.
If it is loose, it can give poor or limited engagement between the plunger and the slidestop (and thumb safety) as it moves around slightly.
Just some thoughts.
