dear fellow shooters,
When i got my STI a few years ago I got the same problems.
I took the gun a part,placed a mag whit one round in it and noticed that the
.40 s&w round hit the slide stop just a bit.
I took the slide stop out and took a bit of whit a belt sander.
The problem never occured again.
The same problem had a friend whit a para-ordnance .45.
I give it the same treatment and also his gun is jam free.
When it occurs again you should look at the slide stop and you will notice that the
slide stop isn't applied propperlie ( it just came up just a millimeter)
I hope I could be helpfull.
greetings from
Joseph