OK, bear with me here.  If there's other info I need to provide I'll get back to you guys.

My first handgun purchase in '98 was an alloy-frame P13.  Pistol ran fine when I bought it.

I took it to a local smith to have a Novak rear sight and an extended slide release installed.  First thing I noticed when I got the pistol back was that he had made a small cut in the frame in order to install the slide release.  I really didn't think anything of it, altho there was a little bit of white metal showing that I wasn't too please with.  Pistol ran fine for a session or two.

After that I started having problems.  I started getting stoppages where the round would fail to feed, with the slide closing on the round with the round halfway out of the mag and the slide closed on it.  I also got stoppages where it looked like the shoulder of the round would catch on the bottom of the feed ramp.  The slide would also sometimes lock with the slide release during firing.  And finally, the pistol started ejecting brass directly into my forehead, which is, at best, distracting.

First thing I thought about was magazines, but I had the same problems with the two factory and two aftermarket mags I own.  I took it back to the local smith who cut the ejector at an angle, I guess to try and get rounds to eject out to the side.  No go, and I haven't been back to him.  I was kinda frustrated with the whole thing and pretty broke at the time so I just tossed the pistol into the back of the safe where it has remained to this day.  I'm now a little wiser and have a few more funds at my disposal and would like to get it up and running again.

Any ideas as to what the problem might be?  Thanks in advance.

He cut the frame to install a slide stop?I would have kicked ars of gunsmith for that one.
As for the possible problem.Sounds to me like the slide stop was not fitted to the pistol.
First make sure the slide will move easily with the slide stop installed.Pop the recoil spring plug out and try moving the slide.It should move with ease.
If it does not the stop needs fitting.The shoulder of the lug that fits behind the slide to retain the slide stop will sometimes need to be worked down a bit or it will fit too snug against the slide causing resistance.Had to do this on my Wilson bullet proof stop.
The slide stop could also be causing the feed issues.Make sure the slide stop lug is not hanging the cartridges as they feed up from the magazine.I had this problem also after installing a wilson bullet proof stop in my Para.
The Para slide stop lug is radiused and very short.All the after market slide stops I have seen are long and pointed.So you have to fit the slide stops sometime,they are not always a drop in fit.
In fact if that dude that work on your gun gave you your original slide stop back...then try it and see if the gun runs.Thats the simple answer...then go from there.