I have a model 14 with a similarly weird problem. My gun is the father of yours, in the same way the model 12 became the 121 and the 8 became the 81. My gun has a release trigger!!! If you rack the gun while holding the trigger back it will fire the instant you release the trigger !!! I have tried another model 14 and that one will not let you rack the action with the trigger depressed. That is how the gun is supposed to work, mine has obviously been victimized by the stupidest Bubba on the planet.
As to your gun, I would suspect it is a slam fire caused by the combination of a heavy firing pin with no retracting spring. Does it do this when you pump the action gently? if it does, something is seriously wrong..If you handload, I would try to find some hard primers. You might try Magnum primers, with a reduction in the powder charge. Magnum primers are a bit harder than standard, or so I have been told.
That "firing pin catch" in the diagram has a spring....I wonder if that is the culprit? If it is weak perhaps that is the problem.
These Pederson designed pump action rifles are quite complicated and difficult to understand sometimes. If you took the bolt apart on yours and got it back together you have my admiration.. Those things are a whore's nightmare to take apart and re-assemble. Good luck anyway. Sorry I couldn't be more helpful.