They finally found that it the chamber was cut too deeply. The firing-pin strike lost energy in driving the whole round deeper into the chamber. Not every time, but just enough to cause those odd ones, like that. 
Which would really show up when those shorter-than-average cases hit the chamber. I've noticed that nearly all of the new brass I buy these days is at (or sometime below) the minimum length. I'm pretty sure it is the same with loaded ammo, but have not checked.
A chamber problem was my first thought as well. Until you stated it was all factory ammo, I would have guessed that maybe you were roll crimping your loads, a big no no as that cartridge headspaces on the mouth of the case Without a firm case mouth / chamber fit, you are using your extractor to hold the cartridge in position, which sometimes allows a round to be driven slightly deeper into the chamber, resulting in a lite firing pin hit. The same symptoms could appear if the chamber is cut slightly deep. Anyway you slice it, a new gun should work, period. I'd send it back.
Larry
I first became aware of this about two years ago when working up loads for my S&W M625. Maybe half of the factory ammo woud not fire without the moonclips, even though it is supposed to (no extractor there to serve as auxillary headspace); poor ammo, I thought.
I bought some new brass to resolve the problem & started digging a bit deeper. I finally clued in on it when I started to trim the new brass & the Lee case trimmer wouldn't even touch the case mouth. Took out the mic & discovered the length issues.
I also checked a variety of new rifle & pistol brass and discovered this was pretty much the norm; didn't seem to matter much what brand it was.
At any rate, it is still possible that the chamber is within specs, though at the "max" end - if the ammo is at the "min" dimension, problems are likely.
