A failure of the slide to lock back on an empty magazine is incredibly common for SigSauer pistols and can almost always be traced to the shooter holding down the slide lock while shooting. If you're holding the gun correctly, you're likely to experience this. The slide lock is poorly positioned. As my shooting and technique has improved I've found that my slide almost never locks back on an empty magazine. I've tried to adjust my grip, but the better my grip gets the more problems I have with this.
That's its own problem. Your weak hand needs to be fully engaged in holding the gun and you want as much of your weak hand contacting the grip as possible. You cannot have that hand too high on the grip.
I have tens of thousands of rounds through three different P series Sigs (four if we're counting frames) and have never had a magazine related malfunction with Sig or Mec Gar magazines. Other than Mec Gar, after market mags are crap. Hard stop. I would replace the recoil spring, although despite it's age, it doesn't sound like your gun has that much wear.
