As i said in a PM to you, your problem is very real and is most likely related to your firing assembly (firing pin group). Could be dirty, gunky, burr, out of spec part or some form of debris in there. Clean well, dissemble if your comfortable with doing that and hope for the best.
You're so right. Pushing the firing pin back manually reveals the roughness at the same point as the trigger roughness - two places near the end of the pull.
Upon opening the assembly I found the major diameter of the pin to have several pieces of green paint from the spring on it. Actually I don't see how the rod inside the spring could be smooth, being plastic and flexible.
Question. Should the forward end of the major pin diameter be swaged? It shows .006 of an inch larger than the main portion for about 1/16 inch of its length.
Also, the major diameter shows no attempt to polish it beyond about a 400 grit, and almost half its length (at the rear) shows what appears to be a fine stone dremel attack about .006 deep and 1/8 inch wide.
Should I try to remove the swage? That seems tricky to do without damaging the part that's already "right." At least without a very accurate lathe.
Should I try to polish the major diameter with a super-fine grit?