Hello. I ran into the same thing with my P9. Kahr kindly sent me another slide stop, stating that it was what caused the problem. It did not solve the problem. My P9 was quite reliable until I used +P ammo. Then, the problems you mention occurred.
I diagnosed the problem myself and consider it a design flaw, but others do not see it that way. Interestingly, I've heard from 2 others today having precisely the same problem.
As I see it, the spring that holds the slide stop down must be "mounted" just right in the polymer frame to offer enough force to overcome the slide stop lever's upward impulse when the slide abruptly stops in its rearward travel and this impulse is greater with the hotter loads. IF mounted properly, all is fine, but if the hole in which one end of the spring fits is a tad large or a fraction off from where it should be, the spring doesn't offer enough downward force to avoid the premature lock back.
If you are familar with the K9 series of Kahr pistols, you are aware that the way in which the slide stop spring's mounted is radically different than the P9. Examination of the two guns' slide stop levers will show differences, too.
While both are made of two pieces, the pin and the lever, on the P9, the lever portion rotates a full 360 degrees. By not silver soldering or otherwise affixing it to the pin as on the K9, the portion of the lever that the weaker P9 spring has to keep down weighs less and therefore, has less upward momentum for the P9 spring to overcome. To me, a possible fix would appear to be a stronger spring, but perhaps there's some reason why Kahr doesn't think so that I'm not aware of, or this may be just what they're doing on newer guns.
I have read that at least one P9 owner sent his gun back to Kahr and that it was fixed 100%. I have not spoken to anyone face to face on that, however.
I do tend to believe that Kahr will do all they can to make their product right, having heard nothing but praise for their warranty work and customer relations.
However, I've kept my K9 which is flawless and gotten rid of the P9 which I couldn't trust, replacing it with a Glock 26.
I much preferred the "feel" and looks of the Kahr P9, but will take the G26's reliability for now.
Good luck, sir. If I learn anything else on this, I'll post it for those having problems with P9s. I suspect that as is common with many "births," there can be "delivery" problems, but feel confident that these will be corrected.
The problem is that it was too late for me and my particular P9, but perhaps not for you and others.
Best.
