I certainly agree it is an extraction issue and that is the place to look. It is a new gun and I would let the factory know that you are having a problem. That said, before I would return it to the factory I would compare the extractor on the new gun with the extractor on the older gun. If you see a major difference, I would get a new extractor (buy two) and change it when you change the extractor spring. What bothers me is that you said the "stuck" case comes out easily which to me means that the extractor never even got a decent grip on it to begin with. Since it works 95% of the time it is hard to believe the extractor itself is bad, but it is possible. I would also make sure that your slide speed is slowing down and not coming fully into battery. I personally do not grease the slide or frame "rails". Since they are full length and you have lots of contact space, I only use synthetic oil on the rails. Make sure they are staying wet and that the hammer contact area has lubrication.
It is difficult to run down a problem that only happens 5% of the time and apparently not consistently. However, you are going to have the most pressure from the magazine rounds on the slide during the first few feeds, so again, make sure that that center rail on the bottom of the slide has lube on it. I know that both of my limited guns will fire out of battery (slide is closed but the barrel is still not locked up). Good luck