To eliminate the slide stop as the cause, remove the slide from the receiver and reinstall just the slide stop. Load a magazine and tap it on something to be certain that all rounds are fully toward the rear.
Now insert the magazine into the receiver while watching the slide stop. See if the bullet of the top round clears the slide stop lug, or scrapes against it. If it clears, nudge the top round forward just a tiny bit and try again. What you want to do is find out how far a round would have to move forward under recoil to cause a problem. If your magazine followers have the dimple, load just one round and then push that round forward to where the case mouth is stopped by the dimple (which is why it's there). Then try the test again. If a round that's properly located at the dimple (not forward of it) makes contact with the slide stop, then it's time to break out the file.
The reason I specified a half-round file is that you want to remove as little material as possible. Using a half-round file (preferably a fairly small one) allows you to contour the lug around the shape of the bullet.
You can also alter the outer portion of the slide stop to increase resistance to being pushed up. I think Brownells sells a spotting drill that's sized to go in through the plunger tube and drill a small detent for the tip of the plunger to engage. You can control the amount of resistance by how deep you drill the depression.