Good point, wogpotter, and I also thought of a gas port setting too tight, but the problem is that those things would cause a problem on every shot, not the third or fourth. I would like to see that rifle and see if there is a point where the hammer can reach the firing pin with the bolt out of battery. I have been trying it with my 49 and can't get it to do anything wrong, but it does look like some wear might allow contact.
Something like a wrong or incorrectly made (repro?) part cannot really be ruled out after so many years and many hands.
I hate it when something like this happens and the quick glib response is "Oh, they fire out of battery all the time" or something like that. "They" shouldn't, and I like to find out what went wrong, not just talk about floating firing pins that don't float or high primers that aren't high or reloads that aren't reloads.